Step 1Heat the broiler.
Step 2Remove the hard core from each tomato, then place the tomatoes in a 13x9-inch metal baking pan. Add water so that the tomatoes are covered halfway.
Step 3Place under the broiler and broil until the skin of the tomatoes is blackened, 15 to 20 minutes.
Step 4Remove the pan from the oven, turn the tomatoes over and repeat, broiling until the tomatoes are blackened on the other side, another 15 to 20 minutes.
Step 5Remove the pan from the oven; reserve the water.
Step 6Place the tomatoes in a blender with the onion, cilantro, cumin, garlic, pepper and salt and puree until almost smooth, but with bits of charred tomatoes showing.
Step 7If the salsa seems too thick, add a little of the cooking water from the tomatoes and blend again.