Preheat and prep the pan. Heat the oven to 375°F (190°C). Lightly oil a 9x13-inch baking dish and set it aside.

Sauté the vegetables. Warm the olive oil in a large skillet over medium heat. Add onion and bell pepper and cook until softened, about 4–5 minutes.

Add the broccoli and zucchini. Cook for another 3–4 minutes until the broccoli turns bright green and begins to soften. Stir in the garlic and cook for 30 seconds until fragrant.

Wilt the spinach. Add the spinach and cook just until wilted. Remove the pan from heat. Season the vegetables with a pinch of salt and pepper. This step helps drive off moisture so the casserole bakes evenly.

Whisk the eggs. In a large bowl, whisk the eggs, milk, salt, pepper, paprika, and oregano until well combined. Avoid over-whisking; you want it smooth but not frothy.

Layer the dish. Spread the sautéed vegetables evenly in the prepared baking dish. If using cooked sausage or beans, scatter them over the vegetables.

Add cheese and eggs. Sprinkle half the cheese over the vegetables. Pour the egg mixture over the top, then finish with the remaining cheese.

Bake. Place the dish on the center rack and bake for 28–35 minutes, or until the center is set and the edges are lightly golden. A knife inserted near the center should come out mostly clean.

Rest and slice. Let the casserole rest for 5–10 minutes before slicing. This helps it set and makes clean squares.

Finish and serve. Top with chopped fresh herbs if you like. Serve warm with a side of fruit, avocado, or a simple green salad.