Bring a large pot of generously salted water to a boil. Cook pasta according to package directions. Reserve at least 1 cup pasta water, drain, and set aside.
In a large high-sided skillet, cook bacon over medium heat until crisp. Transfer to a paper towel–lined plate and crumble.
Drain most bacon grease, add butter, and cook onion 3–4 minutes until tender. Add garlic and cook 1 minute more.
Reduce heat to medium-low. Sprinkle flour over onions and stir to combine. Cook 1 minute.
Stir in pasta water, cream, and parmesan until smooth.
Add bacon and peas. Simmer, stirring, until sauce thickens. Season with nutmeg, salt, and pepper.
Add cooked pasta and toss to coat.
Serve topped with extra parmesan and fresh parsley.
Notes
Reserve extra pasta water before draining! Pasta water is great for loosening the pasta sauce and achieving the perfect texture.
Before adding cream, turn heat down to medium-low to prevent scorching. Stir constantly until slightly thickened.