Bring a large pot of salted water to boil. Prepare an ice bath and place near the stove.

Using a paring knife, cut a shallow x onto the bottom of each tomato. Carefully add the tomatoes to the boiling water and cook until the skins split, 15 to 20 seconds. Using a slotted spoon, remove the tomatoes and place immediately into the ice bath. When they have cooled completely, removed the skins and discard.

Quarter the tomatoes. Set a fine mesh sieve over a bowl and squeeze the seeds and juice into it. Once all the tomatoes have been squeezed, press on the seeds to extract as much tomato juice as possible. Chop the tomatoes and set aside.

Heat about a tablespoon of olive oil in a large skillet set over medium-high heat. Add the onion and cook until softened, 3-4 minutes. Add the garlic and continue cooking until fragrant, about a minute more. Stir in tomatoes and their juices. Cook until sauce reaches a simmer, then reduce heat to medium-low.

Cook, stirring occasionally, until the tomatoes are tender, 20-25 minutes. Add salt and pepper as needed, then stir in the basil. Serve with your favorite pasta.