For sauce, whisk together first 8 ingredients. Bring a large saucepan of water to a boil; remove from heat. Add noodles; let stand until noodles are tender but firm, 3-4 minutes. Drain; rinse with cold water and drain well., In a large skillet, heat 1 tablespoon peanut oil over medium-high heat; saute chicken until no longer pink, 5-7 minutes. Remove from pan., In same pan, saute onion in remaining oil over medium-high heat until tender, 2-3 minutes. Stir in sauce; cook and stir over medium heat until slightly thickened. Add noodles and chicken; heat through, tossing to combine. If desired, top with cucumber slices and chopped peanuts. Serve immediately.