1. Preheat the oven to 220°C, gas mark 7. Cut away the skin from the celeriac and quarter the flesh. Cut each piece into 1cm thick slices, then across into small chips. Put 2 tablespoons of the oil in a roasting tin and put it in the oven to heat up for a few minutes. Once the oil is hot, carefully tip in the celeriac chips, turning them to coat. Season and bake in the oven for 30-35 minutes until pale golden, turning once or twice with a fish slice.
2. Meanwhile, heat 1 tablespoon of the remaining oil in a frying pan and gently fry the onion for 5 minutes. Tip the beans into a bowl and mash well with a fork. Crumble in the rye and sunflower seed bread. Add the onion, caraway seeds and parsley. Season and mix together well. Shape into 4 even-sized burgers.
3. Heat the remaining oil in the frying pan and gently fry the burgers for about 2 minutes on each side until hot and slightly crisped. Transfer to serving plates with the celeriac chips and serve with pea shoots or watercress and the tomato salsa.