Preheat your oven to 375°F (190°C).
In a large bowl, whisk together the eggs, milk, salt, black pepper, and red pepper flakes (if using) until well combined. Set aside.
Heat 1 tablespoon of olive oil in a 10-inch oven-safe skillet over medium heat. Add the Italian sausage, breaking it into crumbles with a wooden spoon as it cooks. Cook until it's browned and no longer pink, about 5-7 minutes.
Add the diced onion to the skillet with the sausage. Cook until softened, about 3-4 minutes.
Stir in the minced garlic and cook for another 30 seconds until fragrant.
Add the chopped roasted red peppers and spinach to the skillet. Cook for 1-2 minutes until the spinach wilts.
Spread the sausage and vegetable mixture evenly across the bottom of the skillet.
Sprinkle half of the shredded cheese and fresh basil over the sausage mixture.
Pour the egg mixture over everything in the skillet. Give the pan a gentle shake to help the eggs settle in between the filling ingredients.
Cook on the stovetop for about 3-4 minutes until the edges start to set but the center is still quite loose.
Sprinkle the remaining shredded cheese and the Parmesan on top of the eggs.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, until the eggs are set and the top is lightly golden. The frittata should be slightly puffed up and not jiggle when you gently shake the pan.
Let the frittata rest for 5 minutes before slicing. This allows it to set up properly and makes for cleaner slices.
Garnish with additional fresh herbs before serving. The frittata can be served hot, warm, or at room temperature.