1. Cook the pasta in boiling water until almost tender.
2. Meanwhile, put the reduced-fat spread, flour and milk in a small saucepan. Set over a medium heat and whisk constantly until the mixture comes to the boil. Reduce the heat, continue whisking until the sauce is thickened and smooth. Stir in all but a teaspoon of the cheese. Season with black pepper.
3. Drain the pasta, mix with the sauce, tuna, sweetcorn and chives, if using
4. Transfer to two ovenproof baking dishes and top with the remaining cheese. If freezing for future use, freeze now, and then defrost overnight in the fridge before cooking.
5. If eating immediately, grill until golden. Alternatively, keep chilled for up to one day. When ready to use, bake at 170°C/150°C fan/gas mark 3 for 30 minutes until piping hot.