Chop the potatoes into large chunks and place in a pot. Fill the pot with water until it covers the potatoes. Add a large pinch of salt. Cover the pot and bring to a boil. Once boiling, remove the lid, lower the heat, and simmer about 10-12 minutes, until fork tender.
Drain the water off the potatoes and add the butter and milk. Smash the potatoes with a potato masher (or a large spoon or ladle). Once you’ve reached the desired rustic consistency, stir in the cheese and mustard. Salt and pepper to taste.
While the potatoes are boiling, add 1 tablespoon of butter to a large skillet over medium-high heat. Place the sausages into the skillet, allowing them to make contact with the skillet first, then add the sliced onions. Brown the “bangers” and onions for about 10 minutes. Add the beer and allow it to reduce for another 10-12 minutes, stirring occasionally.
Mix 1 tablespoon of soft butter with 1 tablespoon of flour together. Remove the bangers and mix the butter mixture into the beer reduction. It should thicken instantly. Now add the Worcestershire sauce and stock to thin out the gravy to your desired consistency. Salt and pepper to taste.
Serve the bangers over the mash, topped with gravy.
Select a beer that is malty, but not too hoppy. Otherwise the gravy might have a bitter note.Leftovers can be kept in an airtight container in the fridge for 3-4 days.