High Fiber Spinach & Mushroom Breakfast Casserole That Actually Keeps You Full Till Lunch (No Sad Salads Required)

You know that 10 a.m. crash where your “healthy” breakfast betrays you? Not today. This High Fiber Spinach & Mushroom Breakfast Casserole is the power move: hearty, ridiculously tasty, and built for staying power.

It’s the kind of meal that makes you feel like you’ve got your life together—even if you’re eating it standing over the sink between emails. Make it once, eat like a boss all week, and still feel light. Who said “healthy” can’t be craveable?

Why This Recipe Works

This casserole leans on fiber-heavy veggies, whole grains, and protein to keep you full and focused.

Spinach, mushrooms, and onions add complexity and depth, while oats and flax quietly pack the fiber punch. A touch of Greek yogurt gives the custard moisture and creaminess without leaning on heavy cream. Everything bakes into a sliceable, meal-prep-friendly slab that reheats like a champ.

In short: elite morning fuel with minimal effort.

What Goes Into This Recipe – Ingredients

• Olive oil – 2 tablespoons, for sautéing.
• Yellow onion – 1 medium, finely diced.
• Baby bella (cremini) mushrooms – 12 ounces, sliced.
• Fresh spinach – 6 packed cups (about 6–8 oz), roughly chopped.
• Garlic – 3 cloves, minced.
• Large eggs – 8.
• Egg whites – 4 (about 1/2 cup) to bump protein without heaviness.
• Plain Greek yogurt – 3/4 cup (2% or whole for best texture).
• Unsweetened almond milk – 1 cup (or dairy milk).
• Rolled oats – 1/2 cup, lightly pulsed (or quick oats).
• Ground flaxseed – 2 tablespoons.
• Grated Parmesan – 1/3 cup, umami magic.
• Shredded part-skim mozzarella – 3/4 cup.
• Fresh parsley – 2 tablespoons, chopped.
• Smoked paprika – 1 teaspoon.
• Red pepper flakes – 1/4–1/2 teaspoon (optional heat).
• Kosher salt – 1 to 1 1/4 teaspoons, divided.
• Black pepper – 1/2 teaspoon, freshly ground.
• Cooking spray – for the baking dish.

Instructions

1. Prep the pan and oven. Heat oven to 375°F (190°C). Lightly coat a 9×13-inch baking dish with cooking spray.
2. Sauté aromatics. Warm olive oil in a large skillet over medium heat. Add onion and 1/4 teaspoon salt; cook 3–4 minutes until translucent.
3. Brown the mushrooms. Add mushrooms; cook 6–8 minutes, stirring occasionally, until they release liquid and edges caramelize.

   Season with another pinch of salt.

4. Wilt the spinach. Add garlic; cook 30 seconds. Add spinach in batches, tossing until wilted and most moisture cooks off, 3–4 minutes. Remove from heat and cool slightly.
5. Blend the custard. In a large bowl, whisk eggs, egg whites, Greek yogurt, and milk until smooth.

   Stir in oats, flaxseed, Parmesan, smoked paprika, red pepper flakes (if using), remaining salt, and black pepper.

6. Combine and layer. Fold the mushroom-spinach mixture and parsley into the custard. Pour into the prepared dish. Sprinkle mozzarella evenly on top.
7. Bake. Transfer to the center rack and bake 28–35 minutes, until the edges are set and the center barely jiggles.

   An instant-read thermometer should read 165°F (74°C) in the middle.

8. Rest and slice. Let stand 10 minutes before slicing. This helps it set and slice cleanly (and saves you from casserole carnage).

Storage Instructions

• Refrigerator: Cool completely, then store slices in airtight containers up to 4 days.
• Freezer: Wrap portions tightly and freeze up to 2 months. Thaw overnight in the fridge.
• Reheat: Microwave 60–90 seconds per slice, or bake at 325°F (163°C) for 10–15 minutes.

  If reheating from frozen, add a few minutes.

Nutritional Perks

• High fiber win: Spinach, mushrooms, oats, and flax deliver serious fiber to support digestion and satiety. Translation: fewer snack attacks.
• Protein-forward: Eggs, egg whites, and Greek yogurt stack up to a balanced, filling meal that actually fuels your morning.
• Smart fats: Olive oil and flax bring heart-healthy fats for hormone and brain support. Not just tasty—strategic.
• Micronutrient-rich: Iron and folate from spinach, B-vitamins from eggs, and antioxidants from mushrooms make this more than just “breakfast.”

Pitfalls to Watch Out For

• Watery veggies: Don’t rush the mushroom browning or spinach wilting.

  Excess moisture = soggy casserole. Cook it off.

• Rubbery texture: Overbaking turns eggs tough. Pull it when the center barely jiggles and let carryover heat finish the job.
• Salt shy: Underseasoned eggs are bland.

  Taste your veg before mixing and season the custard properly.

• Wrong oats: Rolled or quick oats only. Steel-cut won’t soften enough here, unless you like surprise crunch (you don’t).

Recipe Variations

• Veggie boost: Add 1 cup diced bell peppers or zucchini (sauté first to reduce moisture).
• Cheese swap: Try feta for tang, Gruyère for nuttiness, or goat cheese for creamy pockets.
• Herb-forward: Swap parsley for basil, thyme, or dill. Fresh herbs = instant flavor upgrade.
• Spice play: Add 1 teaspoon curry powder or za’atar for a global twist.

  FYI, smoked paprika + cumin = smoky heaven.

• Meat add-in: Fold in 4–6 ounces cooked turkey sausage or chicken sausage for extra protein without overpowering the veggies.
• Gluten-free note: Use certified gluten-free oats and verify your spices/cheese are GF-safe.
• Dairy-free path: Swap Greek yogurt for a thick dairy-free yogurt and use plant-based cheese; choose dairy-free milk.

FAQ

Can I make this casserole the night before?

Yes. Assemble fully, cover, and refrigerate overnight. Bake cold from the fridge, adding 5–10 extra minutes, until the center hits 165°F.

Do I have to use oats and flax?

No, but they boost fiber and help the casserole hold structure.

If skipping, reduce milk by 1/4 cup to avoid a looser set.

How do I keep the top from over-browning?

If the cheese colors too fast, tent loosely with foil for the last 10 minutes. It’ll still melt beautifully without turning into a crispy hat.

Can I use frozen spinach?

Absolutely. Use 10 ounces frozen chopped spinach, thawed and very well squeezed dry.

Skip the wilting step; just add to the cooked mushrooms and onions.

What if I don’t have Greek yogurt?

Use cottage cheese blended smooth, or swap with 1/2 cup sour cream plus 1/4 cup extra milk. You’ll keep the creamy factor without derailing the texture.

How many servings does this make?

About 8 generous slices. If you’re feeding big appetites (or training days), count on 6.

Why is my casserole watery?

Most likely undercooked vegetables or not enough resting time after baking.

Make sure the mushroom liquid evaporates in the pan and let the baked casserole rest 10 minutes before slicing.

Final Thoughts

This High Fiber Spinach & Mushroom Breakfast Casserole is the breakfast that respects your time and your goals. It’s fast to assemble, easy to reheat, and built to keep you satisfied—without feeling heavy. Meal prep it on Sunday and you’ve basically outsourced your weekday mornings.

Tastes great, performs better. Honestly, what more do you want from eggs?