In an airtight container, whisk all of the marinade/sauce ingredients together. Spoon 1/3 cup of marinade/sauce into a freezer size Ziploc bag and whisk in 1 tablespoon sesame oil and 1 tablespoon cornstarch. Add beef and onions to freezer bag. Refrigerate both the meat/onions and the separate reserved sauce for 15 minutes up to overnight (the longer the better!).
When ready to cook, heat 1 tablespoon sesame oil in a very large skillet over high heat. Once very hot and rippling, remove meat and onions from the marinade with tongs and add to pan. You may need to work in two batches.
Break up any clumps in the meat and let cook for 1 minute, without stirring, then stir and cook until beef is browned and almost cooked through, about 1-2 minutes (it will cook more in the sauce). Don’t overcook or it won’t be as tender! Transfer beef to a large plate and cover. Wipe wok with a paper towel.
Add 1 tablespoon cornstarch to reserved sauce and whisk until smooth.
Heat an additional tablespoon sesame oil in the wok over high heat. Add "stir fry" vegetables and saute for 1 minute. Add reserved sauce and bring to a simmer. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook until sauce has thickened, about 1-2 minutes. Add beef back to skillet with vegetables and sauce and continue to cook until meat is cooked through and vegetables are crisp-tender, about 1 minute. Taste and add additional Gochujang/salt/pepper if desired.
Serve over rice or noodles and garnish with scallions (optional).