Combine ingredients for citrus sauce and set aside. Heat a large, heavy sauté pan or wok over medium-high heat until hot. Add 1 T oil. Add beef in a single layer, cook 1-2 minutes until nicely brown on each side. Repeat with remaining beef. Reserve beef on a large platter, cover with foil and keep warm. Add 1 T oil to the pan and sauté mushrooms 3-4 minutes. Add to the beef platter. Add a bit more oil and the bok choy. Sauté 3-5 minutes and arrange on platter. Sauté fennel 2-3 minutes and add to platter. Add 1 T oil, garlic, ginger and white scallion slices and sauté 1-2 minutes. Add the citrus zest and the citrus sauce, cook for an additional 2-3 minutes until bubbling and hot. Drizzle sauce over platter and garnish with sesame seeds and scallion greens. Serve immediately with rice or thin Asian noodles. Serves 3-4.