In a wok, heat 3 ounces of peanut oil. Add beef strips and stir-fry quickly, just to brown meat. Season with soy sauce and sugar, then remove and set aside.
Clean the wok, and heat the remaining 3 ounces of peanut oil. Add scallions, onions, mushrooms, tofu, spinach, and bamboo shoots. Stir-fry quickly, about 2 minutes. Add mirin and noodles and return cooked meat to the mixture. Stir-fry an additional minute or just until hot and well-incorporated.
Remove from wok and serve at once on a preheated dinner plate.