How to Make It
Toss scallops with 1/2 cup of the cornstarch in a bowl. Shake off excess, and arrange scallops in a single layer on a tray or plate. Refrigerate 15 minutes.
Meanwhile, heat oil in a large Dutch oven over medium-high to 350°F.
Whisk together flour, baking powder, 1½ teaspoons of the salt, 1/2 teaspoon of the pepper, and remaining 1 cup cornstarch in a large bowl. Add club soda and egg yolk, whisking until just combined.
Dredge haricots verts in flour mixture. Using a slotted spoon, remove hericots verts from flour mixture, shaking off excess. Carefully place in hot oil, and fry until crispy, 1 1/2 to 2 minutes. Drain on paper towels. Bring oil back to 350°F; repeat procedure with scallions. Sprinkle 3/4 teaspoon of the salt and ¼ teaspoon of the pepper over cooked haricots verts and scallions.
Bring oil back to 350°F. Add chilled scallops; fry until golden brown, 2 to 3 minutes. Drain on paper towels. Sprinkle with remaining 3/4 teaspoon salt and 1/4 teaspoon pepper.
Arrange vegetables and scallops on a platter. Top with parsley, and serve with lemon wedges.