To a large pan, add the potatoes. Fill with water so it covers the potatoes by about 2-inches (5 cm). Bring to a boil. Simmer for 15 minutes or until they feel tender when pierced with a fork. While they boil, cook the bangers.

Add the oil to a heavy, high-sided frying over medium heat. Brown the sausages on all sides, remove and set aside.

To the same pan you cooked the sausages in, add the 2 tablespoons butter over medium heat. When the butter is melted and bubbling add the onions and cook, stirring often until softened and lightly browned.

Sprinkle the flour into the butter and onions and whisk well to combine. Cook for a minute to cook out the raw flour flavor, don't burn. Slowly (to prevent lumps) whisk in the stock until well incorporated with the flour. Whisk in the mustard, tomato ketchup and Worcestershire sauce until well mixed. Bring to a low simmer until thickened. Add salt and ground black pepper to taste.

Turn the heat down to low on the gravy and add in the sausages to heat through. Meanwhile, drain the cooked potatoes into a large bowl. Give them a few minutes to dry out for fluffier mash. Stir in the melted butter, warm milk and salt. Taste and add more salt if needed.

Place mashed potato onto a plate and top with the 2 bangers per person and onion gravy. Serve with peas.