01 - Preheat oven to 350°F. Grease a 9-inch pie dish or springform pan with butter or cooking spray.
02 - Boil spaghetti in salted water until al dente according to package directions. Drain thoroughly and set aside.
03 - Melt butter in a skillet over medium heat. Cook onion and bell pepper for 4–5 minutes until softened. Add garlic and cook for 1 additional minute. Remove from heat.
04 - Whisk together eggs, milk, Italian herbs, salt, black pepper, and red pepper flakes in a large bowl until fully combined.
05 - Add cooked spaghetti, sautéed vegetables, mozzarella cheese, and half of the Parmesan to the egg mixture. Fold gently until evenly distributed.
06 - Transfer the mixture to the prepared pie dish, spreading evenly. Sprinkle remaining Parmesan cheese across the top.
07 - Bake for 35–40 minutes until the center is set and the top is golden brown. A knife inserted in the center should come out clean.
08 - Allow the pie to cool for 10 minutes before slicing. This helps the custard set for cleaner portions. Garnish with fresh basil or parsley.