Lightly boil or steam the diced sweet potato for 7-10 mins, or until tender.
In a large bowl combine all the rest of the ingredients, including the sweet potato once cooked, and gently mix and crush with a potato masher until combined but still chunky. Divide the mixture into 4 and shape into patties.
Fry the burgers with a splash of oil for a few minutes until golden on both sides, then place on a baking try lined with lightly oiled parchment and transfer to the oven to cook at 190˚C (Gas Mark 5, 375˚F) for 10 minutes.
Whilst the burgers are cooking toast 4 burger buns or ciabatta rolls and prepare your fillings.
When the burgers have finished cooking in the oven, assemble with the topping and buns and serve hot. These burgers also freeze well, for up to 3 months, separated by squares of baking paper.
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