Creating the Spinach Mushroom Breakfast Casserole is straightforward. Follow these simple steps:

1. Preheat the Oven: Set your oven to preheat at 350°F (175°C). This ensures an even bake.
2. Sauté Vegetables: In a large skillet, heat olive oil over medium heat. Add the diced onion and minced garlic. Cook until softened, about 2-3 minutes.
3. Add Mushrooms: Toss in the sliced mushrooms. Sauté for another 3-4 minutes until mushrooms are tender.
4. Add Spinach: Stir in the chopped spinach. Cook until wilted, approximately 2 minutes. Remove from heat.
5. Whisk Eggs: In a large bowl, whisk together the eggs, milk, salt, pepper, and paprika until well combined.
6. Combine Mixtures: Add the sautéed vegetables to the egg mixture, stirring gently to combine.
7. Add Cheese: Fold in the shredded cheese of your choice, ensuring an even distribution.
8. Prepare Baking Dish: Grease an 8×8-inch baking dish or a similar-sized casserole dish.
9. Pour Mixture: Pour the egg and vegetable mixture into the prepared baking dish.
10. Bake: Place in the preheated oven and bake for 35-40 minutes, or until the center is set and the top is lightly golden.
11. Cool and Serve: Remove from the oven and let it cool for 5-10 minutes. Slice into squares and serve warm.

Following these steps will lead you to an incredible breakfast casserole that everyone will love!