Heat the oil in a large pan over a medium heat, add the onion and cook for 5 minutes until softened. Add the leftover carrots, parsnips and potatoes and cook for a further 5 minutes, adding the garlic and most of the thyme for the final minute.
Add the chicken stock and the chicken and cook, covered, for 5-10 minutes, until the chicken is piping hot. Stir in the lemon juice. Using a slotted spoon, remove about half the vegetables and chicken to a bowl and set aside.
Using a stick blender, whizz the soup in the pan until smooth (or blend in a food processor), then return the reserved chicken and vegetables to the pan and heat through. Serve in bowls with a dollop of crème fraîche and the remaining thyme scattered on top, with pieces of baguette on the side.