In a large pot over medium heat, soften the onion in the butter for 5 minutes. Add the garlic and cook for 2 minutes while stirring. Add the carrots, broth, milk and lentils. Bring to a boil and simmer for 20 minutes or until the carrots are tender. Season with salt and pepper.
In a blender, purée the soup until smooth. Adjust the seasoning. Serve in bowls. Delicious served with Grilled Gouda and Naan Sandwiches (see recipe).