Peel and finely dice the onion.

Peel and mince the garlic.

Crush the whole tomatoes by hand or pulse briefly in a food processor, reserving their juices.

Chop the basil leaves, reserving a few whole leaves for finishing if desired.

Chop the parsley if using.

Measure the oregano, crushed red pepper flakes, salt, pepper, olive oil, and optional sugar.

Heat the olive oil in a large saucepan or Dutch oven over medium heat.

Add the diced onion and cook for 6–8 minutes until softened and translucent but not browned.

Add the minced garlic and cook for 30–60 seconds, stirring constantly until fragrant.

Stir in the oregano and crushed red pepper flakes, if using, and cook for another 30 seconds.

Add the crushed tomatoes and all of their juices. Stir well to combine.

Bring the sauce to a gentle simmer, then reduce the heat to low.

Simmer uncovered for 30–40 minutes, stirring occasionally, until the sauce has thickened slightly and the flavors have developed.

Stir in the chopped basil during the final 5 minutes of cooking.

Taste and season with kosher salt and black pepper. If the tomatoes are particularly acidic, add ½ to 1 teaspoon of sugar to balance the flavor.

Remove from the heat. For a rustic sauce, leave as is. For a smoother marinara, use an immersion blender or carefully blend in batches until the desired consistency is reached.

Cool before storing. Refrigerate for up to 5 days or freeze for up to 3 months.