Bring a large pot half-full of water to a boil and get an ice water bath (large heatproof bowl) ready. When the water is at a full boil, add one-third of the tomatoes and simmer until some of the skins start to split, about 3 minutes. Transfer the tomatoes to the ice water using a slotted spoon. Let stand for a minute or two, until cool enough to handle, then peel those tomatoes with a small sharp knife and place in a medium bowl to catch any juices. Repeat with the remaining tomatoes in 2 more batches.
Trim, quarter and seed the peeled tomatoes; return to the bowl. Crush the tomato quarters with your hands and reserve.
Place the oil and garlic in wide pot (6 to 8 qts) or a deep skillet over medium low heat. Cook stirring until the garlic is golden but not brown. Add the tomatoes, partially cover and cook over medium-low, stirring often to prevent scorching, until sauce is thickened and tomato bits are very tender, 1-1½ hours. Season with salt and pepper and let cool until ready to use.