Preheat oven to 185c. Place celeriac on a baking tray and spray with a few sprays of fry spray, then cook for 20-25 minutes until it’s softened slightly but not fully cooked, or crisp around the edges.
Heat a large frying pan on low to medium heat. Once hot, add the shiitake mushrooms, part cooked celeriac and rosemary and cook for about 5 minutes until the mushrooms are slightly golden. Add the mixed seeds and cook for a further minute or two, then transfer all of the ingredients to a food processor. Add the nuts, then pulse until everything is evenly combined.
Once combined, split the mix into four evenly sized amounts, then shape into circle burger patties with your hands.
Line your baking tray with greaseproof paper and place the patties on the tray. Cook in the centre of the oven for 25-30 minutes until hot all the way through, firm on the outside but still moist in the middle.