Preheat the oven to 190C/375F/gas mark 5. Line a baking tray with baking parchment.
Mix the ground flaxseed and the water together in a small bowl and set aside.
Place the mushrooms in a food processor and pulse until they are finely chopped.
Heat the olive oil in a wide frying pan over a medium-low heat. Add the diced red onion and cook gently, stirring often, until soft.
Add the chopped mushrooms and cook, stirring regularly, until all of the excess water has evaporated and the mushrooms are beginning to colour a little. Add the garlic and cook for another minute.
Place the walnuts and oats in the food processor and blend until finely chopped. Add the cooked mushroom mixture, the lentils, breadcrumbs, cornflour, soy sauce, miso paste, flax egg, thyme, paprika, salt and pepper.
Pulse until the mixture is fairly finely chopped but still retains a bit of texture.
Give it a good mix, then divide into six and shape each portion into a patty using your hands.
Bake the burgers for about 30-35 minutes, flipping them over halfway through, until firm to the touch and crispy.
Serve with buns and your choice of toppings.