Instruction

1. Whisk together lemon juice and olive oil in a medium bowl until combined. Add fennel, apple, and mint; toss to evenly coat. Season with salt to taste. Cover and chill until ready to use.
2. Stir together mayonnaise and garlic in a small bowl. Cover and chill until ready to use.
3. Pour oil to a depth of 2 inches into a large heavy-duty skillet; heat over medium to 350°F. Whisk together flour, baking powder, and salt in a medium bowl until well combined. Just before frying fish, whisk beer into flour mixture until combined and consistency resembles a thin pancake batter. (A few lumps are OK.)
4. Dip fillets, 1 at a time, in batter, turning until well coated. Remove, and allow excess batter to drip off, leaving a thin film of batter on fish. Using tongs, lay fish in hot oil, and hold in place until batter sets and fish begins to float. Cook, turning occasionally using a fish spatula (or a heatproof tool such as a slotted spatula, slotted spoon, or spider), until crust is golden brown and crisp, and a thermometer inserted into thickest portion of fish registers 145°F, 3 to 4 minutes. Transfer to a paper towel–lined plate; season with additional salt to taste. Repeat cooking process, working in batches as needed, with remaining fillets.
5. Spread both sides of each hamburger bun liberally with garlic mayo. Add lettuce, fish, and a generous amount of slaw to bottom buns. Add top bun, and serve with lemon wedges.