Boil a pan of water. Add the sausages and cook for 5 mins, then add the potatoes for another 5 mins. Drain well and slice the sausages.
Meanwhile, heat the oil in a small frying pan and fry the onion and apple until softened. Add the sliced sausages, potatoes and seasoning, then increase the heat and fry until golden and crispy, mashing the potatoes a little as you fry. If your pan is ovenproof, crack the egg on top and grill for a few mins until cooked to your liking. If not, fry the egg in a second pan.
Mix the grated beetroot, dressing and vinegar together, and serve with the hot sausage hash, fried egg and rocket.