In a medium bowl, whisk together the marinade ingredients. Add beef to marinade and refrigerate for 30 minutes.
Meanwhile, whisk together sauce ingredients and set aside.
Heat the 2 tsp vegetable oil in a wok or skillet over medium high heat until quite hot. Add the beef strips and cook, stirring, until beef is cooked through. Add the green onion and cook, stirring, until the green onion softens and browns, slightly. Add the sauce and cook, stirring, for 1 minute. Add the udon noodles and stir gently to loosen the noodles, then cook, stirring, until the sauce thickens and noodles are well warmed through.
Serve garnished with sliced green onion and sesame seeds.
Be sure to read the "Cook's Notes" in the original post, where I share some valuable tips, options, substitutions and variations for this recipe!