Place your saucepan over medium heat and add the butter. Let it melt completely and foam slightly, which takes about one minute.

Add your diced onion to the melted butter and stir frequently for about three minutes until it softens and becomes translucent.

Stir in your minced garlic and cook for exactly one minute, stirring constantly.

Add the entire can of diced tomatoes, including all the juice.

Stir in the salt, pepper, basil, sugar, and red pepper flakes if you're using them.

Reduce your heat to medium-low and let everything simmer gently for about five minutes.

Pour in your heavy cream and whisk it in thoroughly to combine everything smoothly.

If your gravy seems too thin, mix the cornstarch with water and whisk it into the sauce slowly while the gravy simmers.

Take a spoon, blow on it to cool it slightly, and taste your creation. Add more salt, pepper, or basil if needed.