1. For the caramelised onions, heat the olive oil in a small saucepan and gently fry the onions until they are golden, add a splash of cold water and continue cooking over a gentle heat until golden and softened.
2. Meanwhile, place 1 slice of the rye bread in the food processor and blitz to form fine breadcrumbs, tip into a bowl and set aside. Place the diced venison into the food processor and blitz until it is finely chopped.
3. Return the breadcrumbs to the food processor with the finely chopped shallot and egg yolk. Mix until evenly combined.
4. Using damp hands divide the mixture in half and pat into 2 burgers.
5. Heat the oil in a non-stick frying pan and gently fry for 6-7 minutes on each side or until the burger is cooked through and there is no pink meat.
6. Just before serving, toast the remaining 2 slices of rye bread, crush half an avocado over each, top with a slice of tomato, the venison burger, a lettuce leaf and the caramelised onions. Serve with sweet potato wedges if you want to make this into a more substantial meal.