if you need to par-boil your vegetables bring a small amount of water to a boil, add vegetables, cook for 2 minutes, cover and remove from heat

remove the skin from the fish if necessary and cut into roughly bite size pieces

arrange in 9x13 baking dish and set aside

melt the butter in a sauce pan over medium heat, whisk in the flour and salt

cook and stir for one minute then slowly whisk in the milk

keep whisking it and cook until it begins to thicken

reduce the heat and gradually add all the cheese

taste and adjust for salt and maybe add a pinch of pepper

drain the vegetables and sprinkle them over the fish in the baking dish

pour the cheese sauce over the top

cover the top with the mashed potatoes, no need to spread smoothly, let some of the cheese sauce show through

bake in the oven at 350˚ for 40 minutes or until the sauce is bubbling up over the potatoes in places and the potatoes are starting to turn brown

let it rest 5 minutes or so to let it cool down a bit

You'll probably want to serve this with a nice hunk of bread for soaking up extra cheese sauce.