Poke the steak strips liberally with a fork and toss in a baking dish.
Pour the soy sauce on top and flip the steaks to coat. Let sit in the fridge for at least 15 minutes, covered.
When ready to cook, place the mushrooms in a large bowl, cover, and microwave 4 to 5 minutes. Drain the liquid and set aside.
Combine the dry mustard, water, and sugar in a small bowl and whisk until a paste forms. Season with salt and pepper.
Heat the oil in a large non-stick frying pan over medium heat and pat the steak strips dry.
Place the steaks in the frying pan and brown on all sides, 6 to 9 minutes. Remove meat from pan and transfer to a bowl.