DO-AHEAD TIP: If desired, cook brown rice. Cut round steak lengthwise in half and then crosswise into 1/8-inch thick strips; place in a medium bowl then add 3 tablespoons of teriyaki sauce; toss to coat. In a small bowl, blend together remaining teriyaki sauce, peanut butter, water, ground ginger and crushed red pepper flakes; set aside. Meanwhile, heat the oil in a large non-stick skillet or wok over medium-high heat until hot; add beef (half at a time) and stir-fry for 1 to 2 minutes or until outside surface is no longer pink (DO NOT OVERCOOK!). Add peanut sauce mixture and toss to coat meat. Arrange brown rice evenly on dinner plates and top with meat/sauce mixture; sprinkle with chopped cucumber and serve immediately.