Preheat the oven to 190°C (375°F) and line a large baking sheet with parchment paper.
In a large bowl, mash the (skinned) sweet potato with a fork until fairly smooth. Add the remaining ingredients and mash into a rough consistency that holds together when pressed.
1 large sweet potato, 150 grams cooked chickpeas, 300 grams cooked brown rice, 1 small red onion, 2 cloves garlic, 20 grams fresh parsley, 75 grams walnuts, 2 tablespoons olive oil, 1 tablespoon apple cider vinegar, 2 teaspoons cumin, 1 teaspoon sea salt, 1/2 teaspoon black pepper, 1/2 teaspoon cayenne pepper
Separate the mixture into about 10 equal balls, then use your hands to form them into patties about 2 cm (3/4 in.) thick. Place onto the prepared baking sheet.
Bake for 35-40 minutes, or until lightly golden. The burgers will be slightly delicate right out of the oven, but can be lifted with a spatula. Serve with buns and toppings.
Leftovers keep well at room temperature for a couple of days, and the mixture freezes well before baking. If frozen after baking they tend to get a bit crumbly after thawing.
Notes
If you'd like to take an extra step for more depth of flavour, sauté the onions and garlic for a couple of minutes before adding, and toast the walnuts. It's not necessary but can mellow out the onions and add another layer with the walnuts.