Slice onion, green pepper, and red pepper into straws. Cut the beef into slices across the fibers.

Mix the ingredients for the marinade. Pour the marinade over the beef slices, mix well, and let marinate for 10 minutes.

Boil the spaghetti until tender in a pot of salted boiling water; drain.

In a non-stick pan, fry the marinated beef slices, then transfer to a plate. In the same skillet, saute the onion and bell pepper. After sauteing the vegetables until the onions are cooked, add the beef slices and spaghetti to the skillet. Drizzle with soy sauce and season with black pepper. Stir.

Arrange the cooked pasta on serving plates and serve.