Press clove into onion and place onion in a large saucepan. Add stock, bay leaf, lentils and a pinch each of salt and pepper. Bring to a boil over high heat, then reduce heat to medium and simmer, skimming occasionally, until lentils are tender, 30-35 minutes.
Discard onion and bay leaf, and stir in garlic, thyme and butter. Reduce heat to medium-low and cook, just below a simmer, for 10 minutes, allowing flavors to blend. Season with salt and pepper, then ladle into bowls. Garnish with mint and serve. If desired, soup can also be puréed before serving.