Place beef on a few layers of kitchen towel and drain some blood.
Meanwhile, in a small mixing bowl combine all the sauce ingredients and mix well.
Place the beef in a large mixing bowl, pour the sauce and mix well with your hand. Let it sit for 15 minutes for the meat to soak the flavor.
Heat a skillet over medium-high heat, add some beef (You might need to cook 2-3 batches to cook evenly), onion, and mushroom. Start tossing all together to cook evenly, about 2-3 minutes.
Serve over freshly cooked hot rice and sprinkle sesame seeds to garnish. Kimchi will make a wonderful side dish to serve with.
* To make sea kelp stock, simmer 1 cup of water with a piece of dried sea kelp for 10 minutes.