1. Mix flour, cornstarch, cayenne, garlic powder, and 1/2 tsp. salt in a medium bowl. Add olive oil, then slowly drizzle in 3/4 cup cold water, whisking constantly until mixture is smooth. Cover and chill at least 1 hour.
2. Pour ponzu into a small bowl and sprinkle with green onion. Set aside.
3. Pour 1 in. vegetable oil into a large, wide pot. Insert a deep-fry thermometer; heat over high heat to 375°.
4. Meanwhile, beat egg whites with a mixer until soft peaks form. Using a rubber spatula, fold whites into batter. Transfer batter to a wide, shallow bowl.
5. Drop a few asparagus stalks into batter. One by one, transfer asparagus to hot oil. Cook until light golden brown, turning as needed, about 4 minutes. Drain on paper towels and sprinkle with more salt if you like. Repeat with remaining asparagus. Serve with ponzu.
*Find ponzu in the Asian foods aisle.