British food is at its core simple and hearty. This dish is both, and can be thrown together with no more preparation than opening a beer and slicing some onions. There are many types of sausages that will work in this preparation, but if you're here for something British (and this is a good place to be if you are), then the banger is what you're after. Traditionally, bangers are a high-fat, simply seasoned pork sausage; they get their name from the popping noise they make as you fry them in a pan. If you can't find anything that says 'banger' on its label go for any sort of mild pork sausage—bratwurst would work perfectly.
The term ale describes one of two categories that all beers fall into. Generally speaking, ales use malted barley, brewers yeast, and warmer, top of the tank fermentation. They fall on the sweeter side, and are often a bit darker than their lager brothers. The inherent sweetness of ale works magic with the sweetness of slowly cooked onion, scented with just a bit of thyme. If you have a favorite ale, by all means use whatever is in your fridge, but if you're not normally an ale drinker, I like using a Newcastle brown ale in this dish.
Since this dish is so simple, the ingredients you choose makes all the difference when it comes to the final result. This version is a straightforward, comforting weeknight meal. But if you are looking for something a little bit different, I'd suggest a Cumberland sausage roasted with cider onions.