1. In a medium saucepan, heat oil over medium-high. Add bacon and cook until browned and almost crisp, about 4 minutes. Add onion and cook until softened, 3 to 5 minutes. Add garlic and red-pepper flakes and stir until fragrant, 1 minute. Add tomatoes, breaking them up as you go, and season with salt and pepper. Bring sauce to a boil; reduce to a simmer and cook until slightly reduced, 15 minutes.

2. Meanwhile, in a large pot of salted water, cook pasta 1 minute less than package instructions. Reserve cup pasta water; drain pasta and return to pot.

3. Add sauce and pasta water to pasta and toss to combine; cook over medium-high until sauce thickens and coats pasta, about 2 minutes. Refrigerate 3 cups pasta for a separate dish. Serve remaining pasta topped with Parmesan and parsley.