Chop the onion and saute in a little oil until soft.
Put the flour and milk in a cold saucepan and heat gently, whisking, until it thickens. Season with salt and pepper and a generous dollop of any mustard, The mustard, strangely, makes it taste creamier, and cheesey, even tho there is none in it.
Cook the pasta in salted water until al dente, we don’t want the pasta too cooked or the bake will be soggy.
Mix the onion and the drained pasta into the sauce, add the chicken. Stir it all up and put in an ovenproof dish.
Make up the stuffing as per the instructions on the pack. Form into balls with roughed up edges for more crispy bits and drizzle with a little oil.
When you are ready to eat, bake in a moderate oven for 20 minutes, along with the stuffing balls, until the top browns, or microwave for a few minutes until piping hot all the way through. This will freeze well, so can be stashed away as a lovely home made ready meal