To make the sauce, turn on the broiler, and place a rack 6 inches away from the heating element. Place in a large cast-iron skillet or on a foil-lined sheet the sliced peaches, skin side down, the onion, the garlic, and the jalapenos, skin side up. Place under the broiler, and cook for 5 minutes or until the peaches, garlic, onion, and jalapeños are softened and have begun to blacken. Remove from the oven. When cool enough to handle, peel off the skin from the peaches (don’t worry if it doesn’t all come off) and place in a blender along with the onion, garlic, and jalapeño.