Tatws rhost is Welsh for roast potatoes, which can either mean ordinary roast potatoes or it can refer to a specific dish of bacon and potatoes as follows. Slice some potatoes and place in an iron pot, cover the potatoes with proper thick rashers of bacon, sprinkle with a few chopped spring onions and a bit of water, place pot on open fire and wait until cooked. This was all very well in days gone when open fires and indeed iron pots were present in every home, but rather inconvenient in a world of fitted kitchens and built-in gas hobs.
A modernised version of the dish would suggest that take something like an ovenproof casserole dish, and peel and slice a couple of pounds of potatoes. Arrange a layer of potatoes at the bottom of the casserole, place you rashers of bacon and chopped spring onions on top, then add a further and final layer of potatoes. Throw in half a cup of water, cover the casserole with a lid, place in an oven set to Gas Mark 3 or 175C and cook slowly for two or three hours. (Depending on thickly you have sliced your potatoes and how soft you prefer them to be when cooked.)
Some people would suggest that you should remove the lid for the last twenty minutes or so in order to brown the top layer of potatoes. (For appearance's sake at least.) Some might even suggest that you dispense with the water and add half a pint of a basic white sauce at which point you might rechristen the dish as bacon and spring onion hotpot.