Preheat the oven to 190C | Gas Mark 5. On a baking sheet, prick the sweet potato(es) all over with a knife or a fork. (You will need 200g of cooked sweet potato and I find that two small or one large sweet potato do the job)
Bake for around 45 minutes or until tender. Let cool, then split the tops open with a knife and scoop out the flesh and set aside.
In a medium saucepan over medium heat, bring water to a boil. Add the rice and a large pinch of salt. Bring back to a simmer then lower the heat and cook for around 15 minutes, or until rice is tender. Set aside to cool.
Place 1 tablespoon of olive oil in a medium pan, add the onions and fry over small-medium heat until they are soft and translucent.
Place all the ingredients bar the oil and flour into a food processor and pulse until desired consistency. You can make them as smooth or as grainy as you like. I prefer mine to have a bit more texture to them rather than being completely smooth so I only blend them for a few seconds until all ingredients are roughly combined.
Give the mixture a good stir, then divide in half. Scoop out half of the mixture onto a piece of cling film, then roll and press into a sausage shape leaving enough room for twisting at the ends like a cracker. Repeat with the other half and place in the fridge to firm up for at least 2-3 hours.
Once you're ready to fry your sausages, remove from the fridge and carefully unwrap each sausage. Sprinkle with a little plain flour (this will make them more manoeuvrable and prevent them from sticking). Cut each big sausage into three smaller ones, then gently tap the ends with your finger to smooth out the edges.
Heat a little oil in a medium frying pan and fry the sausages, turning occasionally, until browned on all sides.