In a small bowl, mix the first six ingredients. Remove 1/4 cup mixture to a large bowl; add beef and toss to coat. Marinate at room temperature 10 minutes.
Cook spaghetti according to package directions. Meanwhile, in a large skillet, heat 1-1/2 teaspoons canola oil. Add half of the beef mixture; stir-fry 1-2 minutes or until no longer pink. Remove from pan. Repeat with an additional 1-1/2 teaspoons oil and remaining beef mixture.
Stir-fry water chestnuts and green onions in remaining canola oil 30 seconds. Stir in spinach and remaining hoisin mixture; cook until spinach is wilted. Return beef to pan; heat through.
Drain spaghetti; add to beef mixture and toss to combine. Sprinkle with chili pepper.