1. Strain the Tuna and Microwave for 2-3 minutes (make sure you put Tuna / Other fish into something thats not metal!).
2. Grate Cheese (if making a Cheese sauce over a Parsley sauce).
3. Heat White sauce and add either Cheese or Parsley. (Both if you want!!).
4. Mis the sauce with the fish and put into an Ovenproof dish.
5. Wash, cut and Boil the Potatoes. Once soft mash with Milk (add Butter if you want - can be easier).
6. Spread the Mash over the fish / sauce mixture.
7. Cook in Oven at Gas Mark 6 (or whatever it is for electric oven - I think its 200C / 400F - do check though) until golden brown on top.