Place the tomatoes and garlic in a saucepan, cover with water, and place over medium-high heat. Simmer for about 10 minutes, until the tomatoes are cooked and mushy. Using a slotted spoon or tongs, transfer the tomatoes and garlic to the jar of a blender, along with a cup of their cooking liquid, the toasted chiles de arbol (you can add gradually if you choose), onion, vinegar, cumin and salt. Puree until completely smooth. Taste for vinegar, salt and heat, and adjust accordingly.