Cook rice: I always use a rice cooker, which makes this even easier. Start the rice first, and then turn to the beef and onions.
Take 1 bunch bean thread, put it in a deep bowl, and pour boiling water over them. Cover bowl, and let soak at least 10 minutes, while you proceed with the recipe.
In a saucepan, put the soy sauce, mirin, and sugar. Bring this to a boil, then add the sliced onion and reduce the heat. Let the onions cook until just barely translucent.
Add in the mushrooms, if you’re using them, and let them cook through. Now add the sliced beef, stir, and let simmer until the slices lose their red color.
Drain the bean thread, and add these noodles into the pot. Stir to combine, and serve over rice. Top gyudon with red pickled ginger threads (benishoga).