I never make it with potato ..too sloppy ..mashed potato is OK on top of a fish pie.I make tuna pasta bake.
Cook fusili pasta ... drain .Make a cheese sauce,flake the tuna into the sauce ,add a small tin of sweetcorn,some finely chopped spring onions,finely diced green and red pepper,stir the pasta in and season with fresh chopped parsley and black pepper..put it in a dish ,crumble fresh breadcrumbs or crisps over it and bake for twenty minutes or so until the topping is golden.