Partially freeze pork. Cut pork across grain into thin slices, then into 1-inch-wide strips; set aside. Wrap tortillas in foil and bake in a 350 degree F oven for 5 to 10 minutes or until warm. Spray four 10-ounce custard cups with nonstick coating. Place a tortilla in each cup, pressing to fit. Bake in a 350 degrees F oven for 10 to 12 minutes or until crisp. Remove shaped tortillas from cups; cool.
Combine broth and cornstarch; set aside. Spray a large skillet or wok with nonstick coating. Heat skillet or wok. Stir-fry half of the pork on medium-high heat for 2 minutes or until no longer pink; remove from skillet. Add oil; repeat with the remaining pork. Return all pork to skillet.
Stir in apple, green onion, cilantro, garlic, lime peel, lime juice, salt, and pepper. Cook and stir for 1 minute. Add the cornstarch mixture to skillet. Cook and stir until thickened. Cook and stir for 2 minutes more. Fill tortilla cups with mixture. Garnish with yogurt, if desired. Makes 4 servings.