I usually do this early on the trip - like the first night - to take advantage of fresh ingredients. It uses instant mash, which is a lot better these days than it was in my youth.
Ingredients for 4 people
Instant Mash. Whatever portion size it says on the box - double it.
1 large red pepper
1 large onion
Oil
Tin of Corned Beef
Chop the onion and the pepper into whatever chunks you like. Fry the onion in the oil until soft, then add the pepper and fry for a few minutes more. Add the volume of water that it says on the box and bring to the boil. Chop the corned beef and add to the pot just before adding the instant mash and stir until everything is well mixed. Leave to stand for a few minutes to make sure the mash has absorbed the water then serve. Brown sauce sachets (borrowed from the services on the way to the venue) would go down well.