1. Preheat your oven to 350°F (175°C). Lightly grease a 9×9-inch or similar baking dish with nonstick spray or a thin layer of butter.

2. Heat the olive oil or butter in a skillet over medium heat. Add the diced onion and cook for 3–4 minutes, until softened and translucent. Stir in the minced garlic and cook for 30 seconds, just until fragrant.

3. Add the spinach to the skillet. If using fresh spinach, cook until wilted and most of the moisture has evaporated. If using frozen spinach, make sure it is fully thawed and squeezed very dry before adding; cook for 1–2 minutes to warm through and remove excess moisture. Set the spinach mixture aside to cool slightly.

4. In a large mixing bowl, whisk the eggs until the yolks and whites are fully combined. Add the cottage cheese, milk or half-and-half, salt, black pepper, paprika, dried Italian seasoning, and red pepper flakes if using. Whisk until everything is evenly combined; the mixture will look slightly lumpy from the cottage cheese.

5. Spread the spinach and onion mixture evenly over the bottom of the prepared baking dish. Sprinkle about three-quarters of the shredded cheese over the spinach.

6. Pour the egg and cottage cheese mixture evenly over the spinach and cheese. Gently shake or tap the dish to help the mixture settle into an even layer. Sprinkle the remaining shredded cheese on top.

7. Bake for 30–35 minutes, or until the casserole is puffed, golden around the edges, and set in the center. A knife inserted into the middle should come out without wet egg attached.

8. Remove the casserole from the oven and let it rest for 5–10 minutes to finish setting. Slice into squares and serve warm.