Preheat oven to 300 degrees F. Remove excess moisture from the beef, salt it and sprinkle with black pepper too. Melt the butter in a skillet and start frying the beef for 4-5 minutes on each side. Add 2 sprigs of rosemary.
Put the roast beef in a baking dish, pour in the broth and put 2 more sprigs of rosemary if you like. Roast for 20 minutes (or 12-15 minutes per pound). For the ideal medium rare, the temperature inside the beef must be between 115 F and 125 F. When done cover with foil and set aside to rest for at least 15 minutes.
Heat up the olive oil and start frying the slices of onion over medium heat. When the onion is slightly softened, add brown sugar and continue to cook until the onion is softened - another 7 minutes, stirring.
Ideally it is best to slice this beef after it has been cooled. To do this wait until it has cooled, keep covered with foil and refrigerate for 2 hours. Cold beef is easier to cut into thin slices.
To make baked roast beef slider sandwiches preheat the oven to 360 F. Cut the roast beef into thin slices. Put the bottom of the buns on the 9 * 13 baking dish, and put slices of roast beef on top of that.
Top the roast beef with your cooked caramelized onions. Lay slices of cheese on top of the onions. If you want to add some sort of sauce you could brush mayonnaise, dijon mustard or bearnaise sauce over that. Top with the other half of the buns
Bake in the oven for about 10 minutes or until the cheese has melted and the buns are golden brown. Serve!