Make a small x-shaped incision in the bottom of each tomato. Plunge the tomatoes in a large pot of boiling water for about 30 seconds. Drain and plunge into a large bowl of ice water. Slide off the skins and scoop out the seeds.
In a food processor, pulse the tomatoes and garlic. Stop pulsing while there are still some tomato pieces. Add the oil.
In a saucepan, bring the tomato mixture to a boil, reduce the heat and simmer gently until reduced to the desired thickness, 30 to 45 minutes. Season with salt and pepper.