Preheat the oven to 200°C, fan 180°C, gas mark 6. Put the mushrooms on one side of a baking tray and mist with cooking spray. Wrap the beetroot in kitchen foil and put it on the other side of the tray.
Bake for 45 minutes or until the mushrooms are dried and the beetroot is tender. Leave the beetroot to cool, then rub off the skin and roughly chop the flesh.
Put the beetroot and mushrooms into a food processor with the peas, lentils, rice, parsley, breadcrumbs, egg and garlic.
Season, then pulse briefly until the mixture holds together – it should still be chunky, so take care not to over-process. Shape the mixture into 4 patties and chill them in the fridge for 1 hour to firm up.
Mist a large nonstick frying pan with cooking spray and cook the beetroot patties over a medium heat for 5 minutes on each side, or until browned.
Mix together the mustard and mayonnaise, then spread half of the mixture over the base of each burger bun.
Top with the salad leaves followed by the beetroot patties, remaining mustard mayo, onion and pickles. Sandwich with the remaining bun halves and serve.