First, start by boiling some pasta according to the package instructions. Drain and set aside while you make the sauce. Preheat the oven to 180c.
Melt the butter in an oven-safe dish and garlic. Let it soften gently in the butter for 3-5 minutes until its fragrant. Once the garlic is fragrant add in the flour and whisk to a paste.
Once you've whisked the butter and flour to a paste and cooked it out for about 5 minutes to get rid of the raw flour taste, gently and slowly pour in the milk and vegetable stock. Whisk continuously to avoid lumps in the flour. Simmer gently for about 5 minutes to thicken the sauce. Stir occasionally so the sauce doesn't catch on the bottom of the pan. Once the sauce is smooth, thick, and creamy stir in the frozen peas, drained sweet corn, and chopped parsley. Add the mustard and mixed spice. Stir half the cheese into the sauce and reserve the other half for topping the pasta and baking. Taste and adjust the seasoning.
Drain the tuna and fold it in leaving any large chunks. Toss the pasta in the sauce and make sure that every noodle is coated evenly. Scatter the remaining cheese over the top of the pasta and bake in the preheated oven until the pasta is hot all the way through and the cheese is melted, golde,n and bubbling, about 20 minutes.