Make your burgers: Put a heavy frying pan on a medium heat and add a splash of oil. Once hot, add the chopped mushrooms and thyme and a pinch of salt and pepper. Fry until the mushrooms have released their moisture. Turn off the pan, and leave to cool.
Add the drained beans, garlic, parsley, tahini, 50g of your pumpkin seeds, 50g of your sunflower seeds, chia seeds and dates to the food processor, and blitz until just smooth. Add the quinoa, lemon zest and juice, and cooled cooked mushrooms and give it a tiny whiz, you want it a bit chunky. Firm up the mix in the fridge and then shape into eight patties.
Smother the outsides of the patties in the remaining seeds, reserving some for the buns.
Preheat your oven to 220C/425F/Gas 7 and bake the burgers for 10-15 minutes, until golden brown.
Turn the oven down to 180C/350F/Gas 4. Split your brioche buns in half and brush the tops with the egg yolk and scatter the remaining pumpkin and sunflower seeds over. Pop the tops and the bottoms in the oven for five minutes to warm through.
Now you are ready to assemble your burgers! Spread your bottom bun with a generous dollop of mayo. Layer your beet pickle so that the beautiful pink liquid ripples through the glorious white mayo. Next carefully place your mushroom burger on top of the pickle. Top with a little more mayo, a little more pickle, sprouts and a sprinkling of coriander. Devour!