Preheat the oven: Set your oven to 375°F (190°C). Lightly grease an 8x8-inch baking dish or similar with butter or nonstick spray.

Prep the add-ins: If using onion and bell pepper, warm the olive oil or butter in a skillet over medium heat. Sauté onion for 2–3 minutes until softened, then add bell pepper and cook 2 more minutes. Stir in garlic for 30 seconds. Remove from heat.

Whisk the eggs: In a large bowl, whisk eggs, milk, paprika, oregano, a pinch of salt, and a few grinds of black pepper until smooth and slightly frothy.

Add the chicken and cheese: Fold in the cooked chicken, half of the shredded cheese, and the sautéed veggies (if using). Add half the herbs. Stir to combine.

Assemble: Pour the mixture into the prepared baking dish. Tap the dish gently on the counter to level it. Sprinkle the remaining cheese on top.

Bake: Place on the middle rack and bake for 22–28 minutes, until the edges are set and the center barely jiggles. If you like a deeper golden top, broil for 1–2 minutes at the end.

Rest and garnish: Let it rest for 5 minutes. Sprinkle with remaining herbs. Slice into squares and serve warm.