Heat 3 tablespoons butter in a large pot over medium heat. Add the celery, carrots, turnips and onion and cook, stirring occasionally, until they start browning, about 8 minutes. Add the flour and thyme; cook, stirring, until the flour is incorporated, about 2 minutes. Stir in 4 cups hot water until smooth.
Add the ham and the lentils with their liquid. Increase the heat to medium high and bring just to a boil, then reduce the heat to medium low so the soup is gently simmering. Cook, stirring occasionally, until the soup thickens slightly and the lentils are creamy, about 7 minutes.
Meanwhile, melt the remaining 3 tablespoons butter in a large skillet over medium heat. Add the baguette cubes; sprinkle with the parsley and season with salt and pepper. Cook, stirring, until lightly toasted, about 7 minutes. Serve the soup topped with the croutons.