Bring a pan of water to the boil, add the Jersey Royals and simmer for 15 mins until tender. Drain and roughly crush with the back of a spoon.
Head 2 tsp of rapeseed oil in a frying pan, add the spring onions and garlic, and cook for 3-4 mins until softened. Stir in the potato mixture and set aside to cool.
Once cool enough to handle, add the lentils, feta, tomatoes, parsley, oregano and some black pepper.
With wet hands, shape the mixture into 4 burgers. Put the polenta on a plate and coat the burgers all over.
To make the slaw, combine all the ingredients in a bowl and mix well. Chill until required.
Heat the remaining oil in the frying pan. Cook the burgers for 6-8 minutes until golden on the outside and hot in the middle.
Split the burger buns, fill with the beetroot slices, some avocado, a burger each, and serve with the slaw.