Bring a large pot of water to a boil, salt well, and cook pasta according to package instructions. Reserve 250mL/1c pasta water then drain. Return empty pasta pot to medium heat and add diced bacon. Cook, stirring often, until fat has rendered and bacon is brown and becoming crisp, 7-8min. Remove bacon and the bulk of fat from the pan and drain bacon on towel.
Return pot to heat and add olive oil, diced onions, and a pinch of salt, scraping as the onions cook. When onions have softened, add reserved pasta water, drained pasta, and butter. Stir and toss until emulsified.
Add rendered bacon, peas, and grated parmesan. Remove from the heat. Stir and toss gently until parmesan is melted.
Serve and top with a pinch of grated parmesan and black pepper.