Pre-heat the grill to hot.
Rub the slices of tofu with the groundnut oil. Dust in the cornflour, shaking off any excess. Grill the tofu slices for 6-8 minutes, turning a few times, until golden brown all over.
Meanwhile, prepare the guacamole by mashing together all the ingredients for the guacamole, making sure you keep some texture to it.
Remove the tofu and lightly toast the cut side of the buns for 1-2 minutes until lightly golden. Remove and spread the bottom halves with the guacamole. Top with the tofu, then a few slices of tomato.
Garnish the tomato with a few red chard leaves. Lay the top halves of the burger buns next to the rest of the burger and serve immediately.