Cook bacon in a wide frying pan over medium-high heat, stirring often, just until fat is rendered (3-5 minutes). Add onion; cook, stirring often until soft (about 5 minutes). Stir in garlic and red pepper flakes and cook for 1 minute. Stir in tomatoes with their liquid. Bring to a boil; reduce heat and simmer, stirring occasionally, until slightly thickened.