Preheat broiler to high. Place tomatillos, garlic, onion, and pepper on a foil-lined heavy-duty rimmed baking sheet. Broil without moving until well charred on top side, about 15 minutes. Flip vegetables with a spatula and continue broiling until well-charred and completely softened, about 15 minutes longer. Transfer vegetables to a food processor and process until roughly pureed. Season to taste with kosher salt and allow to cool to room temperature. Fold mixture into guacamole and serve immediately with warm tortilla chips.
