In a large saucepan, combine the lentils with the water, garlic and 1 tablespoon of the thyme and bring to a boil. Cover partially and simmer the lentils over low heat until tender, about 45 minutes. Drain and transfer to a bowl to cool.
Meanwhile, in a medium skillet, melt the butter in the olive oil. Add the carrots, onion and celery and cook over moderate heat until tender. Season with salt and pepper and the remaining 1/2 tablespoon of thyme. Stir the vegetables into the lentils and season with salt and pepper. Serve at room temperature.