Preheat oven to 425 degrees. Heat oil in a large saucepan over medium heat. Add onion and garlic, and season with salt and pepper; cook, stirring occasionally, until softened, about 5 minutes. Add thyme and tomatoes. Bring to a boil, reduce heat, and simmer, breaking up tomatoes with spoon, until sauce is thickened and reduced to about 5 1/2 cups, 20 to 25 minutes.
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