Preheat the oven to 425 degrees.
Scoop the chiles, garlic and onions onto a rimmed baking sheet. Toss them with 1 tablespoon olive oil, then spread them out in a single layer. Place the tomatillos on a separate rimmed baking sheet. Slide both baking sheets into the preheated oven. Roast until the vegetables are soft and slightly charred, about 16 to 18 minutes. Half way through the cooking time, stir the onion mixture and flip the tomatillos. Remove and let cool.
Combine the tomatillos, chiles and garlic in a food processor or blender. Process to a coarse puree. Scrape into a medium-sized bowl. Add the onions, cilantro, and sun-dried tomatoes, stir to combine.
Cut the avocados in half, running your knife around the pit from stem to blossom end and back up again; twist the halves in opposite directions to free the pits and pull the halves apart. Scoop out the pits. With a soup spoon, scoop out the avocado flesh. Place in the bowl with the tomatillo mixture. Mash the avocados with a large fork or old-fashioned potato masher. Season with salt, usually about 1 teaspoon.
Transfer to a serving bowl, cover with plastic wrap directly on the surface of the guacamole and refrigerate until you’re ready to serve. Garnish with sun-dried tomatoes.