Place the butter in a small pan and sauté the finely diced onions for 2-3 minutes over medium heat until translucent. Next, add the finely chopped oregano and stir well. Remove the pan with the onion and oregano mixture from the heat and let it cool to room temperature.
To prepare the bone marrow steak burger patties, cut the rump steak into very fine cubes until it looks like coarse ground beef and place the steak cubes in a mixing bowl. Then, remove the frozen bone marrow from the freezer and cut into four 20 gram slices. Place the four slices back into the freezer and grate the rest of the bone marrow into the bowl of steak cubes.
Add the cooled onions and oregano mixture to the diced steak bowl with the grated bone marrow and mix well. Season the mixture with salt and pepper and form four equal sized burger patties. Place the patties on a plate, cover and freeze for one hour.
To top your bone marrow burger patty. This steak burger with bone marrow topping is mouthwatering to just think about. Crush the peppercorns in a mortar or carefully with the side of your knife and place them on a plate. Next, take the frozen bone marrow slices out of the freezer and roll them in the crushed peppercorns until they are covered.
Preheat the Otto Grill to the maximum level for 3 minutes. Cut open the brioche buns and briefly grill the surfaces on the Otto Grill for toasted buns. Then, place the bone marrow steak burger patties on the grill grate, insert them into the Otto Grill and adjust the height with the grill lever to Meat-O-Meter level 2. Here, you will grill the patties for 1.5 minutes at high heat. Lastly, put the Meat-O-Meter to level 5 and change the heat to low so you can cook the bone marrow steak burger patties for another 2 minutes on each side to a medium rare doneness.
Place the bone marrow slices that are now covered in pepper onto the Otto Grill at high heat and toast them on all sides at the highest level (Meat-O-Meter level 1) for 30 seconds.
Spread mayonnaise on the brioche buns and place one bone marrow steak burger on each bottom half. Then, place one bone marrow slice covered in pepper on top of each patty and top it all off with the top bun half.