Chop bok choy into quarters and roast on a sheet pan with 1 tablespoon sesame oil at 425°F until tender and edges brown (12 minutes); meanwhile, cook ramen noodles according to package directions, drain and set aside.
Mince garlic and whisk together with tahini, soy sauce, rice vinegar, and 1 tablespoon sesame oil to make sauce.
Toss warm drained noodles with sauce and divide among four bowls.
Top each bowl with roasted bok choy and a quarter of the crushed peanuts.