Curried potato soup

Ingredients

• 30 gram butter
• 2 tablespoon vegetable oil
• 2 brown onions, 1 coarsely chopped, 1 thinly sliced, for frying
• 2 clove garlic, crushed
• 1 teaspoon garam masala
• 1/2 teaspoon ground turmeric
• 1 teaspoon ground cumin
• 1 teaspoon ground coriander
• 800 gram coliban potatoes, peeled, coarsely chopped
• 4 cup (1l) chicken stock
• 1 1/2 cup (375ml) water
• 1/2 cup (140g) yoghurt