1. Steam or boil the potatoes until soft and let cool. Meanwhile, roast, peel, and seed the poblanos. Roast the tomatillos and onions until lightly charred.
2. Combine the roasted vegetables and potatoes in a food processor or blender and puree. Add the serrano chile, cilantro, mint, lime juice, ginger, and avocado and puree until smooth, adding milk as necessary to facilitate the blending. Transfer to a bowl or storage container and thin to the desired texture with milk. Add salt and pepper to taste. Chill.
3. Taste the chilled soup for seasoning and correct if necessary. Mix the sour cream and chipotle chile together and drizzle over each bowl soup as a garnish. Note: Normally this soup is served cold, but it can be reheated gently and served warm. If you prefer it less spicy, leave out the chipotle garnish and simply garnish with a sprig cilantro. Serves 8 to 10 inchThe avocado in this soup turns a traditional potato-based soup into something the French would never recognize! Because of the richness of the avocado, we can use milk instead of cream without compromising the texture."