Heat olive oil or butter in a large saucepan (or deep sauté pan) over medium-high heat until melted. Add crushed red pepper flakes and sauté for 2 minutes, stirring occasionally. Add onion and sauté for 5 minutes or until softened and translucent, stirring occasionally. Add garlic and sauté for 2 more minutes or until fragrant, stirring occasionally. Add in the tomatoes and stir to combine.

Continue cooking the sauce, breaking up the tomatoes with a wooden spoon (or potato masher) as you go, until the sauce reaches a low simmer. Reduce heat to medium to maintain a steady simmer. Then let the sauce continue to cook uncovered for about 25-30 minutes, stirring occasionally, until it has thickened** and reached your desired consistency.

Stir in the basil, salt and pepper. Then taste the sauce, and season with any extra salt, pepper and/or crushed red pepper flakes as needed.

Serve immediately. Or let the sauce cool and then refrigerate it in a sealed container for up to 4 days.