Mushroom and celeriac truffle burger

Method

1. Put all the dried mushrooms in a bowl, pour over cold water and soak for 2 hours. Drain (discard the liquid or save to make stock), then squeeze out as much liquid as possible.
2. Heat the oven to 200°C/180°C fan/gas 6. Toss the celeriac pieces in a large baking tray with a glug of olive oil and cook in the oven for 30 minutes until tender. Cool slightly.
3. In a large frying pan (or 2 smaller pans) heat the 100ml olive oil over 
4. a medium-high heat and cook the shallots for 8-10 minutes until light golden brown. Add the chestnut mushrooms, the soaked mushrooms and the garlic, then cook over a high heat. They will release liquid as they cook; the idea is to cook until all the liquid evaporates. Take the pan off the heat, stir in the chopped herbs, then tip onto a chopping board.
5. Finely chop the cooked mushrooms to the size of lentils. Mash the roasted celeriac with a potato masher, then put in a bowl with the mushrooms. Add the breadcrumbs, egg white powder (if using), eggs and truffle oil, then mix together until combined.
6. Divide the mixture into 8 equal patties (about 170g each), then put on a large baking tray lined with non-stick baking paper. Chill in the fridge for 2 hours to set (or leave them overnight).
7. Heat the oven to 180°C/160°C fan/gas 4. Heat a glug of oil in a large frying pan and fry the burgers in batches of 2-3 until there is a little colour on both sides (3-4 minutes total), then put back on the tray and cook in the oven for 15-20 minutes until piping hot throughout. (Alternatively, cook on the barbecue for 10-12 minutes, turning once, until crisp – no need for the oven.) I like to serve this burger in a toasted brioche bun with lettuce, mayonnaise, sliced gherkin and sliced tomato .