Sweet Potato and Lentil Soup

1 T oil
1 clove garlic
1 t salt
2 leeks, sliced
2 t cumin
800g canned brown lentils, drained and rinsed
1 medium sweet potato, 1cm cubes
6 C hot water
½ t vegetable stock
Black pepper
Cook oil, garlic, leeks 5 mins, add salt 5-10 mins. Add cumin 1 min. Add remaining ingredients and bring to boil 20 mins, until lentils soft.