Place a large pot on to boil with 1 liter of water, bring to a boil. When boiling add the pasta to cook.
When the pot of pasta starts boiling again add the vegetables. Cook until the pasta is al dente (firm to the bite).
Strain the pasta and vegetables, put to one side.
Tip about a half cup of milk into a cup. Heat the remaining milk in a saucepan along with the liquid from the can of tuna until almost boiling.
While the milk is heating whisk the corn starch into the half cup of milk.
When the milk is almost boiling add the corn starch slurry and stir until it starts to boil, this will thicken the milk making a white sauce.
Add the onion and stir through the white sauce.
Add the shredded cheese and stir into the white sauce while it is simmering until all the cheese has melted. This is then known as a bechamel or cheese sauce.
Add the tuna, breaking it up with a fork as you go to the size you prefer, some people like small, others prefer large chunks of tuna.
Stir until well combined and just starts to boil again.
To Serve
Potato Topping: Make a mashed potato, then place on top of the tuna mornay, similar to a shepherd’s pie. This makes a complete meal. Works best when cooking in a Dutch oven as the pot.
Cheese Top: Add a thin layer of bread crumbs and additional cheese. Season with herbs of choice or a herb rub. Bake in a casserole fashion in a Dutch oven, this creates a crispy cheese topping.
Stew with Side: Serve the tuna mornay as a stew then add a side such as mashed potatoes or fresh vegetables.