1. Melt butter in a large pot. Add onions, garlic and cayenne pepper and sweat on low heat for 5 minutes. Add potatoes, 500ml water and stock, turn up heat and bring to the boil.
2. Turn down and simmer for 25 minutes until potatoes are soft when pierced with knife.
3. Stir in the cheese, salt and cream and bring back to a simmer. Remove from heat.
4. Blend in food processor until smooth.
5. To serve, reheat, stirring continually in a heavy based pot.