Begin by cooking pasta according to package directions. Before draining the pasta, ladle one cup of water from the pot and set aside. Then drain well.
Next in a large skillet, fry the bacon over medium heat until it is browned. Transfer the cooked bacon to a paper towel to remove excess grease. Remove all but 2 tablespoons of bacon fat from the skillet.
Then add the onions to the skillet and cook slowly over medium heat until softened. Stir the garlic and peas into the skillet. (Note: For canned peas, remove the skillet from heat. For frozen peas, continue to cook for 3 minutes, stirring as needed.)
Return the bacon to the skillet and add the cooked pasta. Pour the reserved pasta water and lemon juice over the dish and stir together until well coated.
Stir the parmesan into the mixture. Top with black pepper to taste.