Combine the tomatoes, chiles and garlic in a medium saucepan, cover with water and bring to a boil over medium-high heat. Reduce heat to medium and simmer for 10 to 15 minutes, until tomatoes are completely cooked and mushy and the chiles are plumped and rehydrated.
Transfer the tomatoes, chiles and garlic to a blender, add the onion, salt and pepper, and purée until completely smooth.
Heat the oil in a medium saucepan over medium heat. Once hot, pour in the tomato sauce, cover partially and simmer for 5 to 6 minutes, stirring occasionally, until the flavors and color have deepened and the sauce has thickened slightly. Serve hot, warm, room temperature or chilled. The salsa will keep for 5 days in the refrigerator.