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. Sauté onion and bell pepper for 4–5 minutes until softened. Add garlic and cook 1 additional minute. Remove from heat.
04 - In a large mixing bowl, whisk together eggs, milk, Italian herbs, salt, black pepper, and red pepper flakes until well combined.
05 - Add cooked spaghetti, sautéed vegetables, mozzarella cheese, and half of the Parmesan to the egg mixture. Fold gently until evenly distributed.
06 - Pour the mixture into the prepared pie dish, spreading evenly. Sprinkle remaining Parmesan cheese across the top.
07 - Bake for 35–40 minutes until the center is firm and the top is golden brown. A knife inserted near the center should come out clean.
08 - Let the pie cool for 10 minutes before slicing to allow it to set. Garnish with chopped fresh basil or parsley and serve warm.