Combine all of the ingredients in the slow cooker, except the oil, sage and creme fraiche, give a stir then cover and cook on Low for 6 hours until the vegetables are tender and breaking apart.
Meanwhile, warm the oil in a frying pan over a high heat. Add the sage, lower heat to medium and cook carefully until crispy. Remove and place on kitchen paper to drain.
If desired, blend the soup in a food processor or using a handheld stick blender until smooth. Reheat in a clean pan if necessary then ladle into warmed bowls. Place a sage leaf on top to garnish and a dollop of creme fraiche.