Slice the beef thinly against the grain. This ensures the meat stays tender. Set it aside.
In a small bowl, mix together the soy sauce, sesame oil, garlic, and ginger. Stir well to combine. Set the sauce aside.
Heat a large skillet or wok over medium-high heat. Add a little sesame oil to the pan, then add the sliced beef. Stir-fry for 3-4 minutes, or until the beef is browned and cooked through. Remove the beef from the skillet and set aside.
In the same skillet, add a little more sesame oil if needed. Add the sliced bell pepper and onion (if using) and stir-fry for another 2-3 minutes until they’re slightly softened.
Add the fresh spinach to the pan and cook for another minute until it wilts down.
Return the cooked beef to the skillet with the vegetables. Pour the sauce over everything and stir to coat evenly. Cook for an additional 1-2 minutes to let the flavors combine.
Return the cooked beef to the skillet with the vegetables. Pour the sauce over everything and stir to coat evenly. Cook for an additional 1-2 minutes to let the flavors combine.