1/4 cup chicken stock (or water, but stock is preferred)
3 tablespoon gochujang
3 tablespoon tamari
1 tablespoon mirin
Green onion, sliced, to garnish
Heat a large sauté pan over medium. Add one tablespoon of the oil and the onion. Cook one minute, stirring often, then add the garlic and cook for another minute.
Add the sliced steak, season with salt, and cook two minutes. Transfer all pan contents to a plate, set aside for now.
Add two tablespoons of the oil to the pan and add the mushrooms, Brussels sprouts, and the two tablespoons of water. Cook, covered, for three minutes, stirring often. If the pan becomes too dry simply add an additional tablespoon or two of water. Remove all to the plate with the steak.
Add the remaining oil to the pan and add the udon noodles. Sauté until softened and pliable, using tongs to help move them around. Meanwhile, measure the water, gochujang, tamari, and mirin in a measuring cup and stir well to combine. Pour the liquid into the pan with the noodles, add the mushrooms and other ingredients, and toss altogether to incorporate. Sauté a few minutes until hot and incorporated. Transfer to a serving dish, garnish with green onion, and serve immediately.