Bring a pot of water to a boil and cook the noodles according to package directions. Add the bok choy for the final 2 minutes, then drain both.
Meanwhile, heat the neutral oil in a large skillet over medium-high heat. Brown the beef for 7 to 9 minutes, breaking it into fine crumbs and cooking to 160°F; let the last minute pass without stirring so some edges crisp.
Stir garlic and ginger into the beef for 30 seconds. Whisk tahini, soy sauce, vinegar, chili oil, sugar and warm water in a bowl until pourable.
Reduce the skillet heat to low. Add the drained noodles, bok choy and sesame sauce and toss gently for 1 minute until evenly coated.
Divide among bowls, keeping some crisp beef crumbs on top, and add the scallions.