Pop your pasta in salted boiling and cook until al dente (take a min or so off cooking, you want it a touch hard). Drain just before adding to the sauce.
Meanwhile, in a large deep pan over medium heat melt in 4 tbsp butter. Whisk in 4 tbsp flour to form a roux, then gradually add in milk, whisking as you go to avoid lumps. Whisk in veg stock and simmer for a few mins until the sauce thickens slightly.
Stir in dijon mustard, salt, white pepper and onion/garlic powder, then turn off the heat and stir in the cheese. Once the cheese has melted, stir in the drained pasta, then gently toss in tuna and sweetcorn (don't be too aggressive or the tuna turns to mush).
Pour into a baking dish, then level off with a wooden spoon. Top with breadcrumbs, a good pinch of salt and pepper and a liberal coating of oil spray. Bake in the oven for 20mins at 200C/390F, or until golden and crispy on top.
Finish with fresh chives, then serve up and enjoy!