Heat the olive oil in a skillet over medium heat; cook the onion and garlic in the hot oil until tender, about 5 minutes. Stir the tomatoes, red pepper flakes, vinegar, and sugar into the onion and garlic mixture; bring to a boil, reduce heat to low, and simmer until the flavors blend to your liking, 45 to 60 minutes. Stir the basil into the sauce; season with salt and pepper.
