In a medium saucepan combine tomatillos and the water. Bring to boiling; reduce heat. Simmer, uncovered, for 5 to 7 minutes or until tender, stirring occasionally. Using a slotted spoon, transfer tomatillos to a food processor or blender; cool slightly. Add 2 tablespoons of the cooking liquid.
Add avocado, green onions, cilantro, sour cream, jalapeno pepper, and salt to the food processor or blender. Cover and process until slightly chunky. Transfer to a covered container; chill for 1 to 24 hours. Serve with grilled meat or poultry.