From Mariquita Farm
You can first cook the onion and tomatillos in a small amount of water, then proceed with the recipe, and include the water when blending.
1/4 cup Onion: (any color); chopped
1/4 cup fresh cilantro; chopped
1/4 teaspoon salt
1/2 lb husked tomatillos; Cut Into Halves
1 fresh serrano chile, seeded
Place all ingredients in food processor workbowl fitted with steel blade or in blender container, cover, and process until well blended.
Makes about 1 1/4 cups sauce.