DO-AHEAD TIP: Marinate beef strips overnight or for at least 3 hours (see recipe). In a large bowl, whisk together first 4 ingredients (vinegar through oil); add steak strips and toss to coat thoroughly; cover and refrigerate overnight or for at least 3 hours. Heat a little olive oil in a large skillet or wok over medium-high heat; add beef strips and marinade; cook and stir for 7 to 12 minutes or until beef strips are cooked to desired level of doneness (do not overcook!). Arrange chopped spinach on dinner plates and top with steak strips (the heat will perfectly wilt the spinach).