Heat 2 inches of neutral oil in a large Dutch oven over medium-high to 360°F. Whisk together flour, cornstarch, baking powder, and 1 teaspoon salt in a large bowl. Whisk in club soda until mixture is smooth and the consistency of thin pancake batter. Let batter stand 5 minutes.
Working in batches, dip mango wedges, halloumi pieces, scallion pieces, and lemon slices in batter, letting excess drip off; add to hot oil. Fry, turning occasionally, until golden and crispy, 1 to 3 minutes depending on size. Transfer to a wire rack set over a baking sheet. Sprinkle fritto misto with remaining 1 teaspoon salt. Serve with chile oil, for dipping.
Melon-y, bright Loire white.