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 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, and half of the Parmesan to the egg mixture. Fold gently to distribute evenly.
06 - Transfer mixture to the prepared pie dish. Sprinkle remaining Parmesan cheese evenly over the top.
07 - Bake for 35–40 minutes until the center is firm and the top is golden brown. A knife inserted in the center should come out clean.
08 - Let the pie rest for 10 minutes before slicing. This allows the custard to set for cleaner portions. Garnish with chopped fresh basil or parsley.