In a large pot of boiling salted water, cook pasta until al dente. Reserve 1 cup pasta water. Drain pasta; return to pot.
While pasta is cooking, heat a large skillet over medium-low. Add pancetta, and cook until browned and crisp, about 15 minutes, stirring occasionally. Using a slotted spoon, transfer pancetta to a paper-towel-lined plate to drain, and set aside (leave fat in skillet).
Increase heat to medium. Add onion and garlic to skillet; season with salt and pepper. Cook until onion begins to soften, 5 to 6 minutes, stirring occasionally. Add tomatoes (with juice); cook, breaking them up with a spoon, until sauce has slightly thickened, 5 to 8 minutes. Add sauce to pasta in pot; toss, adjusting consistency with pasta water if necessary. Serve pasta topped with pancetta and Pecorino.