Toss beef with 2 tablespoons teriyaki sauce. Cook spaghetti according to package directions; drain.
In a large skillet, heat 1 tablespoon oil over medium-high heat; stir-fry beef until browned, 1-3 minutes. Remove from pan.
In same skillet, heat remaining oil over medium-high heat; stir-fry coleslaw mix and onion until crisp-tender, 3-5 minutes. Add spaghetti and remaining teriyaki sauce; toss and heat through. Stir in beef. If desired, sprinkle with cilantro.