Turn on the broiler and place a rack 5 inches away from heating element. Line a cast-iron skillet or baking sheet with foil and place the Hatch chiles, jalapeños, tomatoes (seed side up), and garlic on the skillet. Cook under the broiler for 7 minutes, and then remove the skillet from the oven. The garlic should have light brown spots on it. Remove it from the skillet and place in a blender. Turn over the Hatch chiles and jalapeños (leaving the tomatoes as they are), and return the skillet to the oven.
Continue to broil the chiles and tomatoes for 7 to 8 more minutes or until nicely charred. After this time, remove the skillet from the oven. Place the tomatoes in the blender, and put the Hatch chiles and jalapeños in a paper sack or plastic food-storage bag, close it tight and let the chiles steam for 20 minutes.
To capture any juices and other flavorful bits, add 1 cup of water into the foil-lined skillet, swirl it around, and then pour it into a glass measuring cup. The water should be a light brown color much like the color of weak tea.
After the chiles have steamed, remove from the bag and rub off the skin. Remove the stems and seeds from the chiles and add them to the blender. Add 1/4 cup of the skillet water to the blender and then pulse on low until everything comes together. Depending on how thick you want it, add more skillet water until it’s your desired thickness. Stir in the salt, taste, and make any adjustments.
This is a terrific salsa as is, but you can give it a brighter flavor by adding cilantro and lime juice, if you like. And you can serve it warm or let it cool, it’s good either way. Store it in the refrigerator, and it should keep for about a week.