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 neutral oil in a large skillet over medium-high heat. Brown beef for 7 to 9 minutes, breaking into fine crumbs, until 160°F. Leave unstirred for the last minute to crisp the edges.
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.