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.
Don't judge me, but I always use the Colman's packet mix (well, two actually). I really like the taste of it!
I also use the huge pasta shells you can get in Tesco (can't remember the brand) as loads of the tuna and sauce collects in them- yum! Then top it off with grated cheese and crisps...