Remove husks from tomatillos and wash well to remove sticky residue. Slice in half and place on a baking sheet with the onions. Drizzle with oil and toss to coat. Roast at 450°F for 20 minutes or until tender. Set aside to cool.
Place into a food processor along with the remaining ingredients and process until smooth.
Notes
Use within 2 days. To maintain freshness and color, store in a sealed container with a piece of plastic wrap pressed directly on its surface. This will help prevent oxidation, which is what causes the avocado to turn brown. Makes about 2 cups