Preheat the oven to 180 degrees celsius.
Cook the pasta and frozen vegetables according to packet instructions. Once cooked, drain and rinse under cold water. Note: I did this in two stages, cooking the pasta first, then refilling and boiling water in the same pot to cook the vegetables. Meanwhile, make the creamy sauce by heating a medium saucepan over a medium heat. Add the margarine and allow to melt.
Add the flour and stir until combined with the melted butter. Using a whisk, gradually pour in 1 cup of the milk and stir constantly until the milk is combined with the butter and flour mixture.
Add the remainder of the milk, vegetable stock and mustard and continue whisking. As the milk starts to heat the sauce will thicken. Make sure to whisk constantly to avoid sauce becoming stuck on the bottom of the saucepan.
Once the sauce has thickened, remove from the heat and stir through the parmesan cheese and 1/2 of the grated cheese.Tip: the sauce is thick enough when the sauce sticks to the back of a spoon and you can draw a line through the sauce without it filling back in. Combine the pasta, tuna, vegetables and sauce in a large dish and stir well to combine.
Transfer to a large baking tray and sprinkle the remainder of the grated cheese over the top.
Bake for 15-20 minutes or until the cheese on top is melted and golden.