Place flour and salt in a large bowl; stir in boiling water until dough forms a ball. Turn onto a floured surface; knead until smooth and elastic, 4-6 minutes. Place in a large bowl; cover and let rest for 30 minutes., Divide dough into 8 portions; roll each portion into an 8-in. circle. Sprinkle each with 1 tablespoon green onion. Roll up jelly-roll style; holding 1 end of the rope, wrap dough around, forming a coil, pinching to seal. Flatten slightly. Roll each coil to 1/8-in. thickness., In a large cast-iron or other heavy skillet, heat 1 tablespoon canola oil. Cook pancakes, 1 at a time, over medium-high heat until golden brown, 2-3 minutes on each side, adding more oil as needed; keep warm. , In the same skillet, heat sesame oil over medium-high heat. Add onion, garlic and ginger; cook and stir until tender, 5-6 minutes. Add the remaining ingredients; cook until heated through, 3-4 minutes longer. Spoon about 1/4 cup beef mixture down center of each pancake. If desired, sprinkle with sliced green onions. Roll up tightly and serve.