Bring a large pot of water to a boil; cook the noodles according to package directions. Drain; let cool completely.
Bring 3 cups water to a boil in a medium saucepan; boil the bok choy 30 seconds. Drain; rinse well. Let cool; coarsely chop.
Whisk together the peanut butter, oil, soy sauce, vinegar, garlic, sugar, and red pepper flakes. Put the sauce, noodles, and bok choy into a bowl; toss well. Sprinkle with seeds. Refrigerate in an airtight container until ready to serve, up to 2 days.