Leek soup without cream | Futureater

Ingredients

• 2 tablespoon olive oil
• 1 leek (thinly sliced)
• 1 small turnip (cleaned and chopped)
• 1 large potato (peeled and cut to small cubes)
• 1/2 litre stock or water ((add half of a stock cube if using water))
• 1 teaspoon butter
• a few walnuts
• salt