- Cook the pasta in a pan of boiling water for 10-12 minutes or according to packet instructions.
- 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).
- Remove from the heat and gradually pour in the milk, whisk until smooth after each addition.
- 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 and then return to the pan and fold in the tuna and vegetable sauce with the chives.
- Divide among bowls to serve.