Heat the olive oil in a pan on high until it's shimmery. Sauté the onions in the oil, stirring frequently, until they're soft and shiny, about three minutes. Add in the garlic and stir everything to combine, then cook for an additional two minutes. Add in the basil, red pepper flakes, salt, and sugar. Stir that stuff, then add the butter. Continue to cook and stir for about a minute.

Add half of the can of the crushed tomatoes, scraping the pan so nothing gets stuck. Turn the heat down to low and add the remaining tomatoes. Stir in 1/4 cup water to loosen the sauce, plus a little more if it's still too thick.

Let the sauce simmer for at least. 10 minutes or as long as an hour. Don't forget to give it an occasional stir to keep it loose. Serve over your favorite pasta.