Cook pasta according to package directions. Reserve 1 cup of the pasta water. Drain pasta; keep warm.
Heat oil in skillet over medium heat. Cook pancetta until fat renders, about 3 minutes. Add onions and cook until they begin to release moisture, 4 to 5 minutes.
Add garlic and cook until fragrant, about 30 seconds.
Stir in tomatoes, peas, parsley, Parmesan cheese, and cooked spaghettini. Add the pasta water to create a sauce. Add pepper and red chili flakes to taste. Increase heat until mixture begins to simmer and thicken, stir, and remove from heat.