Step 1

Cook spaghetti in a large pot of boiling water until just tender, 8 to 10 minutes or according to package directions. Drain and refresh with cold water. (Skip this step if using leftover spaghetti.)

Step 2

Meanwhile, heat oil in a large ovenproof nonstick skillet over medium heat. Add onions and cook, stirring occasionally, until golden, 10 to 12 minutes. Transfer to a bowl and let cool slightly. Wipe out the pan.

Step 3

Whisk eggs and milk in a large bowl. Stir in the onions, Parmesan, parsley, basil, salt and pepper. Add the spaghetti.

Step 4

Coat the pan well with cooking spray and place over medium heat. Pour in the egg mixture and distribute evenly in the pan. Cook until the underside is golden, turning the pan around on the burner occasionally to ensure even cooking, 3 to 5 minutes.

Step 5

Invert a large platter over the pan, grasp the platter and pan with oven mitts and carefully turn over. Lift off the pan and spray it again with cooking spray. Slide the frittata back into the pan and cook until the bottom is golden. Slide the frittata onto the platter. Garnish with tomato, if using.