Cook the pasta in plenty of salted water according to the instructions on the packet. When cooked, drain but reserve some of the cooking water, and toss the pasta in some oil to stop it from sticking together.
Place a 30cm ovenproof pan over medium heat and when hot add the reserved oil from the tuna. Add the diced onion and garlic, season with a generous pinch of salt, and saute until translucent. Add some extra olive oil if you feel it needs it.
Add the carrot and saute for a few minutes to soften, then add the spinach leaves and cook until wilted. You can pop a lid on the pan to speed up this process but it doesn't take long. Push the vegetables to one side of the pan.
Melt the butter into the empty side of the pan then add the flour and stir to make a paste. Cook for 1 minute, then gradually add the stock and milk to make a sauce, stirring all the time to avoid lumps. Once the sauce is bubbling away stir through the vegetables and allow to simmer for a couple of minutes.
Add the peas, corn, tuna and a generous grinding of black pepper, then stir through and heat the mixture. Taste and adjust seasoning. Keep warm whilst you prepare the breadcrumb topping.
Preheat the grill to high.
For the breadcrumb topping, combine the breadcrumbs, parsley and parmesan in a bowl then add the oil and rub it in. It should feel like damp sand - add extra oil if you feel it needs more moisture. Sprinkle this over the filling and place the tuna bake under the grill for 5 minutes or until golden brown.
Serve with a crisp green salad.