1. Heat the oven to 220°C/200°C fan/gas mark 7. Put the grated potato in a clean muslin or tea towel and squeeze out the excess liquid.
2. Heat half the oil in a small, non-stick frying pan and fry the spring onion for 3–4 minutes until soft and golden.
3. Mix the onion and sweet potato with one egg then season well with ground black pepper.
4. Shape the sweet potato mixture in four round rosti ‘cakes’ and place on a foil-lined baking sheet.
5. Put the mushrooms and tomato halves on a second foil-lined baking sheet and brush with the remaining oil. Season with pepper and thyme.
6. Put rosti onto upper shelf of oven and mushrooms and tomatoes underneath. Roast everything for 30 minutes until rosti are golden and tomatoes and mushrooms cooked.
7. About five minutes before end of cooking time, poach eggs to your liking. Serve on the rosti with mushrooms and tomatoes.