Steam the sweet potato, or pop it in a pan of water, bring to the boil then simmer, for about 10 minutes or until soft.
Meanwhile put the fish in a microwave-safe dish, add the milk, dot the butter on top and cover. Microwave on high for about 2 minutes until cooked. Alternatively, poach the fish in a saucepan with the milk and butter for about 6 to 7 minutes.
Mix together the fish (including the milk it’s poached in), sweet potato and cheese, then purée until smooth.
Why not try substituting the sweet potato for broccoli?