Preheat the oven to 425°F. While the oven is preheating, lightly coat the tomatillos, garlic, onion, and serrano pepper with olive oil. Place them on a parchment paper lined baking sheet, and season with salt.
Place the baking sheet in the oven on a middle rack, and allow the ingredients to roast for 20 minutes or until lightly browned.
Remove from the oven and place the ingredients into a food processor (being sure to use all the juices from the tomatillos. Add the avocado, salt, cumin, cilantro, and lime juice, pulse till texture becomes creamy. Taste for seasoning and add more salt to taste.