Whisk cornstarch and water to make slurry.
Combine soy sauce, sake, mirin, ketchup, apple juice, sugar, and pepper in pot. Heat on medium 8 minutes.
Whisk in slurry, simmer 1 minute until thickened. Set aside.
Pat steak dry, season generously with salt and pepper.
Heat wok on high with 1 tbsp oil until smoking.
Sear steak 3-4 minutes per side for medium-rare. Remove and rest 10 minutes.
Add remaining oil to wok, cook mushrooms with pinch of salt 4-5 minutes until browned.
Add scallions, cook 1 minute until bright green. Remove vegetables.
Slice rested steak against grain into bite-sized pieces.
Melt butter in wok, add garlic, cook 30 seconds.
Return steak and vegetables to wok, toss with garlic butter.
Add teriyaki sauce, toss 1 minute until glazed and heated through.