Dice the peeled potatoes into large equal size pieces and place it into a large pot of cold salted water. Place the pot over high heat and bring to a boil. Cook until a knife can easily pierce the potatoes. Drain the potatoes and place it into a mixing bowl.

Heat the milk and butter in a saucepan until hot. Meanwhile, mash the potatoes to desired texture. Mix in the hot milk and butter mixture. Season with salt and pepper. Set aside in a warm area.

Melt the butter in a saucepan over medium heat. Once melted, add the sliced onions and cook for 8 minutes until onions are soft. Reduce the heat to low and cook for another 15 minutes.

Sprinkle the flour over the onions and stir to combine. Cook on medium heat for 3 minutes.

Whisk in the stock and bring to a simmer. Add the Worcestershire sauce, salt, and pepper. Simmer until it thickens. Taste for salt and pepper.

Heat a cast iron pan over medium heat. Add the oil and once hot, cook the sausages on both sides until golden brown and cooked through. This should take around 15 minutes.

Serve the bangers on top of the mash potatoes and gravy.