01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper, then arrange bell pepper, zucchini, red onion, and cherry tomatoes on it. Drizzle with 1 tablespoon olive oil and season with 1/4 teaspoon salt, 1/4 teaspoon black pepper, and dried oregano. Toss to ensure even coating.
02 - Roast the vegetables on the baking sheet for 18 to 20 minutes, stirring once halfway through, until tender and caramelized.
03 - Lower the oven temperature to 350°F.
04 - In a large bowl, whisk together eggs, whole milk, remaining 1/4 teaspoon salt, and crushed red pepper flakes until smooth. Fold in the roasted vegetables, baby spinach, crumbled feta, and half of the grated Parmesan cheese.
05 - Heat 1 tablespoon olive oil in a 10-inch oven-safe skillet over medium heat. Pour in the egg mixture and gently stir for 1 to 2 minutes until edges start to set.
06 - Sprinkle the remaining Parmesan cheese evenly over the top. Transfer the skillet to the oven and bake for 15 to 18 minutes, or until the frittata is puffed and fully set in the center.
07 - Allow the dish to cool slightly before slicing and serving.