Preheat oven to 350.
Add the olive oil to a 10 or 12" ovenproof skillet. Saute the red onion and garlic cloves for 2-3 minutes. Season with a little salt and pepper. (If using fresh chopped herbs, add those here.) Add the diced tomatoes, and spinach. Cook until the spinach melts into the sauce.
Add the leftover spaghetti noodles and combine.
In a mixing bowl, whisk together the eggs, milk and some salt and pepper.
Pour the egg mixture over top of the spaghetti noodles and sauce. Cook on the stovetop for about 3-5 more minutes, or until the eggs on the edge begin to set.
Place in the oven and bake at 350 for 15-20 minutes, or until the eggs have set in the middle. Sprinkle with Parmesan cheese just before slicing.
Slice and serve warm with fresh fruit and/or bread.