Preparation Combine avocado, cilantro, green onion, and serrano chiles in processor. Puree until smooth. Add 1/4 cup water and 21/2 tablespoons lime juice and process to blend. Add more water to avocado mixture by tablespoonfuls if needed and blend until thick sauce forms. Transfer to small bowl; season to taste with coarse salt and pepper and more lime juice, if desired. DO AHEAD: Can be made 2 hours ahead. Cover and chill.
Preparation Combine avocado, cilantro, green onion, and serrano chiles in processor. Puree until smooth. Add 1/4 cup water and 21/2 tablespoons lime juice and process to blend. Add more water to avocado mixture by tablespoonfuls if needed and blend until thick sauce forms. Transfer to small bowl; season to taste with coarse salt and pepper and more lime juice, if desired. DO AHEAD: Can be made 2 hours ahead. Cover and chill.