Ooh now this is a lovely lovely recipe. My Mum's been making it as long as I can remember, it tastes gorgeously cheesey and also ticks the comfort food box.
Here we go then....
Slice up a pile of potatoes into slices maybe 0.5cm thick.
Slice up a couple of onions.
Grate a huuuuuuuge pile of cheese.
Take a casserole dish and fill it with layers of potato, onion and lots and lots of cheese. When the dish is full, pour milk on until the milk comes up to around 1cm below the brim.
Put it in the oven and bake for.... 3 - 4 hours. It seriously does take that long to cook as it's really dense and you need the potato to get really soft. If you don't have that long, then stick it in the microwave for 10 or 15 minutes first to get it hot through, then you can put it in the oven for around 1 hr and it should be fine. Check it's cooked by sticking a knife in the middle, if the potato is properly cooked then it should go in really easily.
Dish up piles of the cheesey goodness and enjoy!