How to Make It
Combine steak and 1 tablespoon fish sauce in a shallow dish. Cut 1 onion in half lengthwise. Grate half of the onion. Add onion pulp to steak; toss to coat. Cover and let stand at room temperature for 30 minutes.
Cut remaining 1 1/2 onions into 1/4-inch-thick vertical slices. Cut each asparagus spear diagonally into 3 pieces. Combine sliced onion, asparagus, bell pepper, and oil; toss to coat. Heat a large wok or stainless steel skillet over high heat. Add vegetables to pan; stir-fry 5 minutes or until crisp-tender. Add remaining 1 1/2 teaspoons fish sauce to pan; stir-fry 30 seconds. Remove vegetable mixture from pan; keep warm.
Scrape onion pulp off of steak. Return wok to high heat. Add steak to pan; cook 3 minutes on each side or until desired degree of doneness. Place steak on a cutting board; let stand at least 5 minutes. Cut steak across the grain into slices. Serve with vegetables.