Mix all the ingredients in a large bowl using your hands. Cover the bowl with cling film and chill until needed (for at least 1 hour, if time allows).
With wet hands form 8 balls , then flatten these slightly between your palms. Brush the patties on both sides with some oil, the griddle, fry or barbecues them to your liking. You can put a slice of cheese on top of them if you like.
Meanwhile, cut the buns in half, then toast, barbecue or grill them.
Brush the bottom half the buns with mustard mixed with mayonnaise, then add a layer of lettuce, tomato slices, avocado, grilled pepper etc. Put the putties on top with a slice of onion or a spoonful of caramelized onion. Finally, cover the patties with the top half of the buns. Serve immediately.
Alternatively serve the patties with French fries and/or a salad.