Fry the onion gently in a saucepan until soft. Scrape out on to a plate until cooled.
While the onion cools, shred the turkey in a food processor. Combine the turkey with the nuts, sage, onion and egg. Shape into 4 burgers and pop in the fridge for 10 minutes to firm up. Shred the Brussels in the food processor.
Take a 5cm circular cutter and remove the centre section of each patty. Replace this with sausagemeat (you can cook this turkey offcut ) Wipe out your frying pan from early and put over a high heat. Fry the burgers for 4 minutes each side, turning frequently until cooked through. Pop into a 100C oven while you cook the sprouts.
Fry the spouts for 3 minutes, tossing regularly, until starting to colour. Add the nutmeg and season to taste.
Toast the buns on the cut side and brush with melted butter. Add the turkey burger, a dollop of cranberry and finally the Brussels Sprouts. Serve with salad.