In a large stock pot, bring water to a boil. Add a generous amount of salt and stir. Add penne and cook for 8 to 10 minutes, until al dente. Reserve 2 cups pasta water (don't forget to do this!).
Cook bacon in a large skillet over medium-high heat. Remove slices to a paper towel-lined plate and set aside. Reserve bacon fat in pan.
With bacon fat in pan, add onion and sauté for 5 to 7 minutes, until translucent. Add chili flakes, and stir in tomato paste. Cook for 30 seconds.
Add 1 1/2 cups of pasta water and pasta. Stir well until all pasta is coated with the sauce.
