If you like spicy foods, add up to 2 tsp. chopped chipotle chile in adobo sauce.
To make Soup: Cover mushrooms with 2 cups boiling water in bowl. Let stand 30 minutes. Drain, and reserve liquid. Remove and discard stems, then thinly slice caps. Set aside.
Heat oil in pot over medium-high heat. Add onion, and sauté 7 minutes, or until soft. Add lentils, chipotle, 1/2 cup mushrooms, reserved mushroom liquid, salt and 5 cups water. Reduce heat to medium low, and simmer, partially covered, 45 minutes. Purée in blender or food processor. Return to pot, and stir in milk, vinegar and butter.
To make Croutons: Heat oil in skillet over medium heat. Add garlic and remaining mushrooms, and cook 1 minute, or until browned and crisp. Transfer to bowl. Add bread cubes to pan, and sauté 3 minutes, or until browned. Toss croutons with mushrooms. Ladle soup into bowls and sprinkle with croutons and chives.