1. Preheat oven to 400°F. Warm an ovenproof nonstick skillet over medium-high heat. Add bacon and cook, stirring, until crisp, about 5 minutes. Remove and drain on paper towel-lined plate. Pour off all but 2 tablespoons fat from skillet and add leeks. Cook, stirring, until softened, about 5 minutes. Return bacon to skillet, add spaghetti and stir.
2. Season eggs with 1/2 tsp. salt and 1/4 tsp. pepper. Whisk in milk. Pour into skillet and stir. Cook, lifting sides of frittata with a spatula to let eggs flow underneath, until lightly browned around edges, about 5 minutes. Sprinkle Parmesan on top, transfer to oven and bake, uncovered, until firm in the center and lightly browned on top, about 7 minutes. Use a spatula to loosen frittata and slide onto a work surface. Cut into wedges and serve.