DO-AHEAD TIP: Marinate steak overnight or for at least 4 hours (see recipe).
In a small bowl, whisk together the first 7 ingredients (soy sauce through black pepper); Place steak pieces in a large zipper-topped plastic bag; add marinade, seal bag and turn several times to coat steak with marinade. Refrigerate overnight or for at least 4 hours.
Heat the oil in a large skillet over medium-high heat; add marinated beef (discarding marinade and bag) and cook until it reaches the desired level of doneness.
Serve over chopped spinach