Any sliced vegetable can be turned into tempura, but sweet asparagus at the height of spring is hard to top. Folding fluffy egg whites into the batter gives this coating its delicate crispness.
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.
Pour ponzu into a small bowl and sprinkle with green onion. Set aside.
Pour 1 in. vegetable oil into a large, wide pot. Insert a deep-fry thermometer; heat over high heat to 375°.
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.
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.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.