Heat the oil in a frying pan. Add the onion and sweet potato, cook over a medium heat for approx. 10 mins., allow to cool slightly.
Blitz the chickpeas and walnut kernels in a food processor, transfer to a bowl. Mix in the sweet potato and panko breadcrumbs, season. Knead by hand to form a compact mass. Divide the mixture into 8 portions, shape into flat round burgers.
Mix the tomato, pepper, coriander and shallot with the oil, season.
Brush the mushrooms and aubergine slices with oil.
Charcoal/gas/electric grill: Cover and grill the mushrooms and aubergines over/on a medium heat (approx. 200 °C for approx. 5 mins. on each side. Place the patties on the grill, cook alongside the vegetables for approx. 5 mins.
Coat half of the mushrooms and aubergine slices with guacamole, top with the patties, salsa and coriander leaves. Place the remaining mushrooms and aubergine slices on top, gently press together.