Today's recipe for Welsh Onion Cake is a traditional Welsh recipe, where the humble potato takes centre stage along with onions and butter. Welsh Onion Cake, or "teisen winwns" and "teisen nionod", as it is known in Wales, is classic comfort food that is cheap and easy to make. (If you live in North Wales, where I live, you may know this recipe as "cacon nionod") It's a simple recipe of layered potatoes and onions, with butter, that is baked in a cake tin, or an enamel pie tin, until it is…