Preheat the oven to 180°C/gas mark 4. Place the beans, avocado, onion, miso, tomato paste, smoked paprika and cayenne in a food processor and blend until smooth. Mix in the cooked brown rice and breadcrumbs.
Form into 5 or 6 balls, then pat out slightly. Tip some extra breadcrumbs into a shallow bowl and coat the burger patties in them. Place on an oiled baking tray and spritz with spray oil (or drizzle a bit of oil over the patties). Bake for 25 minutes, flipping them halfway through.
Mix all the salsa ingredients together in a bowl and set aside.
Fill the burger buns with the baked burgers, salsa and some baby spinach to serve.