Preheat the oven to 180°C. Slice 1 onion into half rings and quarter 20 mushrooms. Cut slices of the blue cheese (about 3 slices of cheese per burger). Bake the onion and mushrooms until golden. Put the 4 O Mundo Hamburger Buns in the oven for 5 min. Heat a frying pan with a little oil and sear one side of the 4 beef patties. Turn the patty around and sear the other side. Add the blue cheese slices immediately onto the burgers and let it melt. Take the O Mundo Buns out of the oven and cut them open. Add mayonnaise and ketchup, the watercress and arugula. Add the beef patty and top it with the onion and mushrooms.