Mix all ingredients for sauce including soy sauce, brown sugar, sesame oil, sriracha and cornstarch, in a small bowl until smooth. Set aside.
Bring a large pot of water to a boil.
While waiting for the water to boil, heat a wok or large deep skillet over medium-high heat and add 1 tablespoon of vegetable oil.
Add the beef and cook until no longer pink, then remove from the pan to a plate and cover with foil to keep warm.
Drain any liquid from the pan then add remaining one tablespoon of oil, garlic and vegetables. Meanwhile, drop the noodles in the boiling water and cook according to package directions. (5-9 minutes for al dente, depending on noodle used)
Cook the vegetables until they begin to soften, then add the beef back to the pan. Drain the noodles and add them.
Toss to combine, then pour the sauce over top. Cook while tossing 2-3 minutes until heated through and sauce has slightly thickened.
Add sliced scallions and toasted sesame seeds and serve hot.
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