1. Preheat oven to 375°F and grease a 9×9-inch baking dish.

2. Heat 1 tablespoon olive oil in a skillet over medium heat.

3. Add 1 pound breakfast sausage and cook until fully browned, about 6–8 minutes. Transfer to a plate and drain excess grease if needed.

4. In the same skillet, add 1/2 cup diced onion and cook for 3 minutes until softened.

5. Add 2 cloves minced garlic and cook for 30 seconds until fragrant.

6. Add 1 cup sliced mushrooms and cook for 4–5 minutes until tender and moisture has evaporated.

7. Stir in 1 cup chopped spinach and cook for 1 minute until wilted. Remove from heat.

8. In a large bowl, whisk together 8 large eggs, 1 cup whole milk, 1/2 teaspoon salt, and 1/2 teaspoon black pepper until smooth.

9. Spread the cooked sausage evenly in the prepared baking dish.

10. Layer the sautéed vegetables evenly over the sausage.

11. Sprinkle 1 1/2 cups shredded cheddar cheese evenly on top.

12. Pour the egg mixture evenly over everything.

13. Bake for 30–35 minutes, until the center is set and a knife inserted comes out clean.

14. Let rest for 10 minutes before slicing into 6 equal portions and serving.