Meanwhile, heat the butter, flour and milk in a saucepan ... ovenproof dish (a lasagne dish is ideal). Flake the tuna on top of the pasta and broccoli. Pour the cheese sauce over the pasta mixture ...
Heat the butter in a frying pan over a medium heat ... Season, to taste, with salt and freshly ground black pepper. Add the onion, tuna and cooked pasta to the cheese sauce and stir until well ...