Mix all beef, salt, and pepper togeather. Place beef between plastic wrap and with a rolling pin, roll out until beef is about 1/4 inch thick. Remove plastic from the top of beef and cut into 5x5 squares. In a large skillet melt butter and place a thin layer of chopped onions in bottom of skillet. Place beef sqaures on top of onions and cover with lid. While beef is cooking place rolls in microwave to warm bread. Put mustard on bottom of bun and when burger is done place burger on bun and top with one slice pickle. Serve hot.