Meanwhile, in a large skillet combine the pearl onions, 1/2 cup water, butter, and a pinch of salt over high heat and bring to a boil. Cover and reduce heat to medium-low and simmer until onions are tender, about 5 minutes. Uncover and bring to high, cooking until the water evaporate. Add in the mushrooms and cook until browned and all the liquid is evaporated, about 5 minutes. Remove the vegetables from the pan and add it with the meat. Pour in 1/4 cup of water into the skillet and scrape with a wooden spoon, adding to the beef.