Take out a mixing bowl. Thoroughly mix Soy Sauce (1/2 cup), Sugar (1/4 cup), Mirim (1/4 cup), Minced Garlic (1/4 cup), Water (1/2 cup), Minced Ginger (1/2 teaspoon!) and Sesame Oil (3 Tablespoons).
Add-in your thinly sliced beef and let the meat marinate for at least 30 minutes. If you have time, let it marinate for 1-2 hours. Or even better, if you leave it overnight in the refrigerator!
Cut onion (1/2 piece) into thin slices. Then cut the stem portion off the enoki mushroom and give it a quick rinse under water. Split the mushrooms into smaller bundles with your fingers.
After the meat has marinated, take out a frying pan. Put it on medium-high heat and place in some oil. Once its hot, use a pair of tongs and place the meat in - don't dump in the marinade, only the meat. Then add in the onion pieces as well.
Once most of the marinade in the pan has cooked away and your onions have turned translucent, add-in the enoki mushrooms. Stir everything around for 30 seconds. Turn off heat and plate.