3 tablespoons olive oil
1 cup diced white onion
1 serrano chile, stemmed, seeded, and diced
3 garlic cloves, minced
Salt, for seasoning, plus 1 teaspoon
4 firm ripe avocados, halved, pitted, peeled and mashed
4 cups chicken broth
1/4 cup fresh lemon juice
1/4 cup chopped fresh cilantro leaves
2 cups water
1 teaspoon freshly ground black pepper
1/2 cup Mexican sour cream or creme fraiche, whisked to soften*
Crostini or croutons, for serving, optional
*Mexican sour crema is called cream or crema acida and can be found in Latin markets