First make the relish. Heat the olive oil in a saucepan, add the onions and cook over a medium heat, stirring occasionally, for 20–25 minutes until soft and golden. Meanwhile, soak the cranberries in the vinegar. Add to the onions with the cranberry sauce and 2 tablespoons water and cook for 10 minutes until thickened and jam like. Season with salt and pepper and leave to cool.
Meanwhile, heat the olive oil in a frying pan, add the leek and thyme and cook over a medium heat, stirring frequently, for 5 minutes. Leave to cool.
Combine the leek mixture, Cheddar, fresh breadcrumbs, ricotta, mustard and salt and pepper in a bowl. Stir in the egg and mix together to form a soft dough. Shape into 12 sausages and roll each one in the dried breadcrumbs.
Heat a little sunflower oil in a frying pan, add the sausages and fry over a medium heat, turning frequently, for 10 minutes until golden and cooked through. Serve immediately with the relish.