Bring a pot of water to a boil; prepare a large ice-water bath. Cut a shallow X in the bottom of each tomato; place in pot and blanch 20 seconds. Use a slotted spoon to transfer to ice bath; let cool completely. Peel and discard skins.
Core, halve, and squeeze tomatoes to remove seeds and pulp. Transfer flesh to a food processor. Add oil, cumin seeds, pepper flakes, and salt. Pulse just until a chunky sauce forms (about 8 times). Transfer to a bowl. Stir in garlic. Refrigerate, covered, at least 8 hours and up to 3 days. Bring to room temperature and remove garlic before serving.