Place onion and mushrooms in a food processor and process until finely chopped. Add mince, breadcrumbs, herbs, tomato purée, salt and pepper. Process in bursts for a few seconds at a time, until mixture binds together but still has some texture.
Mould handfuls of the mince mixture into burger patties using lightly floured hands.
Using a large frying pan or griddle, cook the burgers for 12 to 15 minutes, turning once, until evenly cooked.
Serve with relish and salad, in burger buns or chapati.