1 To make the sauce, melt the butter in a saucepan and stir in the flour. 2  Cook for 1 min, then gradually stir in the milk to make a thick white sauce. 3  Remove from the heat and stir in all but a handful of cheese. 4  Drain the pasta, mix with the white sauce, tuna, sweetcorn and parsley, then season.ethod. 1  Heat oven to 180C/fan 160C/gas 4. Boil the pasta for 2 mins less time than stated on the pack. 2  To make the sauce, melt the butter in a saucepan and stir in the flour. 3  Cook for 1 min, then gradually stir in the milk to make a thick white sauce.