A perfect make-ahead lunch, these noodles—made with a sweet-and-spicy sauce of peanut butter, sesame oil, soy sauce, and crushed red-pepper flakes—can be prepared and refrigerated for up to week.
Bring a large pot of water to a boil; cook noodles according to package directions. Drain; let cool completely.
Bring 3 cups water to a boil in a medium saucepan; boil 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 sauce, noodles, and bok choy into a bowl; toss well. Sprinkle with seeds.
Udon are thick Japanese noodles.