Combine and mix the flank steak, rice wine, pineapple juice, soy sauce, red wine vinegar, green onions, lemongrass, and garlic cloves in a zip-top plastic bag. Seal the bag and marinate in the refrigerator 30 minutes, turning once. Remove steak from bag, reserving marinade.
Place steak on a rack and place under the broiler. Allow to cook 4-6 minutes on each side until desired degree of doneness. Let steak rest for 5 minutes. Cut steak diagonally across grain into thin slices; keep warm.
While steak cooks, combine the reserved marinade with the tomato paste and mustard in a medium saucepan over medium-high heat. Stir well with a whisk and add the beef stock. Bring to a boil and cook until reduced to 2 cups. Season to taste if needed. Serve the sauce with steak.