Method

1. Cook the pasta in a pan of boiling water for 10-12 minutes or according to packet instructions. 
2. Melt the butter in a small over over a medium to low heat. Stir in the flour and cook for 1 minute, stirring quickly with a wooden spoon to make a smooth glossy paste (roux).
3. Remove from the heat and gradually pour in the milk, whisk until smooth after each addition.
4. 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.
5. Cut the green pepper in half and remove the core and seeds, then using a small sharp knife, cut into dice.
6. Drain the tuna and sweetcorn and fold into the sauce with the diced green pepper.
7. Drain the pasta into the sink in a colander and then return to the pan and fold in the tuna and vegetable sauce with the chives.
8. Divide among bowls to serve.