Peel the potatoes and cut them into equal, medium sized pieces. Place them in a saucepan and cover them in cold salted water.
Bring them to the boil and simmer until soft enough to mash.
In the meantime, heat a drizzle of oil in a large, non-stick pan. Add the onion and fry for 7 minutes on a medium heat, stirring regularly.
Add the sugar and soy sauce and simmer for 3 minutes. Set the onions aside and grate the cheese.
Drain the potatoes and mash them with a fork, followed by a potato masher until smooth.
Add the onions, butter and grated cheese to the potatoes. Season with salt and pepper and an extra dash of soy sauce.
Give it a really good stir until thoroughly combined. Place the potato in an oven proof dish, gently smooth it down until evenly spread.
Top with grated cheddar cheese and bake for 25 minutes or until golden brown.
Serve with sausages and veg
