Preheat the oven to 180°C (conventional) or 160°C (fan-forced). Grease one medium-sized oven-proof casserole or baking dish, or 6 individual 1-cup (250 mL) oven-proof ramekins.
Cook the pasta according to the directions on the packet and drain it.
In a large pot over a medium heat, melt the butter and add the onion. Sauté it gently until it becomes soft.
Sprinkle in the flour and stir to coat the onion well.
Pour in a half-cup of milk and stir it until it is mixed in and starting to form a thick, smooth sauce. Add another half-cup and repeat. Add the remaining milk and cook for 1 minute more, stirring continuously.
Remove the sauce from the heat and add in the mustard, cheese, and AdVital Powder. Stir the sauce until the cheese has melted and the sauce is smooth.
In a large bowl, gently combine the cooked pasta and vegetables. Add the tuna, breaking up any large lumps, and fold the mixture gently to avoid it becoming mushy. Fold in the sauce and season the mixture to taste with salt and pepper.
Transfer the mixture into the baking dish or divide it between the ramekins.
Sprinkle the top with the cheese, and bake it for 25 minutes (or until it becomes golden).