Preparation
The Method
Cook the penne in a pan of boiling water with a pinch of salt, if using for 10-12 minutes or according to packet instructions.
Meanwhile, melt the butter in a small pan over a medium to low heat. Stir in the flour and cook for 1 minutes, stirring quickly with a wooden spoon or heatproof spatula to make a smooth glossy paste (which is called a roux). Remove from the heat and gradually pour in the milk, whisk with a balloon whisk until smooth after each addition. Season lightly with salt, if using and plenty of freshly ground black pepper. Bring to the boil, then stir in the mustard or tomato puree until evenly combined. Reduce the heat and simmer gently for 5 minutes, stirring occasionally, until smooth and thickened.
Cut the green pepper in half and remove the core and seeds, then using a small sharp knife cut into dice. Drain the tuna and sweetcorn and fold into the sauce with the diced green pepper. Drain the pasta into the sink in a colander, then return to the pan and fold in the tuna and vegetable sauce with the chives, if using. Divide among wide rimmed bowls to serve.
Prepare Ahead
This tuna and vegetable sauce could be made up to 3 days in advance and stored in an airtight container in the fridge. Alternatively you could cook the pasta for a little less time so it still has some bite and put into a suitable ovenproof dish. Bake for about 30 minutes from fridge cold at 180C / 350F / gas mark 4 and cover with tin foil if you think that the top is browning too quickly. It would also be frozen for up to 1 month and defrosted thoroughly on the bottom shelf of the fridge before baking as could the sauce on its own.