In a stockpot, melt butter over medium heat. Add onion, garlic, and carrot, and saute until tender, about 5 minutes. Add lentils, bay leaf, tomatoes, salt, pepper, and 5 1/2 cups water. Bring to a boil, reduce heat, and simmer until lentils are tender, about 40 minutes.
Meanwhile, heat oven to 425 degrees. Toast bread cubes on a baking sheet, turning occasionally until golden brown, about 7 minutes.
Add escarole to soup, and cook for 5 minutes more. Adjust seasonings, and serve in four bowls topped with croutons and olive oil.