Put 1kg of potatoes, peeled and cut into chunks, in a pot of salted water. Bring to a boil and cook for about 15 minutes, until very soft.
Meanwhile, heat ½ tablespoon of oil in a large skillet. Brown 8 sausages all over until they reach 160°F / 71°C, then set aside.
Leave about 2 tablespoons of fat in the skillet, adding 25g of butter if needed. Cook 1 large sliced onion until soft, then stir in 2 minced garlic cloves.
Stir in 3 tablespoons of all-purpose flour. Stir in about a third of 2 cups of low-sodium beef stock to make a smooth paste, then gradually add the rest and simmer, stirring, until the gravy thickens.
Stir in ¼ teaspoon of dark soy sauce, ¼ teaspoon of salt, and ½ teaspoon of black pepper.
Drain the potatoes well and mash with 40g of unsalted butter, ⅓ cup of milk, and ⅓ teaspoon of salt. Serve the sausages on the mash with the onion gravy.
