In a medium saucepan, melt butter over medium heat. Toss in leeks and cook until soft, ~3 minutes. Add water, peas, and salt, and bring to a boil. Reduce heat, cover pot, and cook until peas are tender, 10-15 minutes (5-7 for frozen peas). In a food processor or blender, purée peas using as much cooking water as needed for a smooth soup, at least 1½ cups. Add more liquid if desired. Return to pan, stir in heavy cream, and reheat. Serve hot and garnish with a dollop of crème fraîche, sprinkled mint, and black pepper.
