Preheat oven to 425°F. Heat the olive oil over medium-high in a medium-sized oven-proof skillet. Add in the fennel seeds, red pepper flakes, onion, and garlic. Cook until onion and garlic are fragrant and tender, about five minutes. Add in the tomato and cook until some of the water is released, about 2 minutes.
Meanwhile, whisk together the milk, eggs, Parmesan, salt, and pepper until light and frothy.
Add the leftover pasta to the tomato mixture, and stir until evenly distributed. Spread the mixture evenly along the bottom of the skillet.
Pour the egg mixture over top of the spaghetti mixture—do not stir. Reduce the heat to medium-low, and cook until the edges of the frittata are solid, but the top is still liquidy, about 5 minutes.
Sprinkle top with herbs, and place frittata in the oven to finish setting up. Frittata is done with the top is no longer liquid and it begins to brown. Remove from oven, and let cool for five minutes before slicing.