1) For the mayonnaise: In a blender, combine all ingredients and process until smooth. Taste and adjust seasoning as needed. Store covered in the refrigerator until ready to use.
2) For the chips: Heat oven to 90C. In a large cast iron skillet, pan, or Dutch oven, heat vegetable oil over medium-high heat to 180C
3) Peel sweet potatoes and cut lengthwise into 1.5cm planks. Cut planks lengthwise into 1.5cm strips. Place strips in hot water with vinegar while oil finishes heating. Line a plate with paper towels and set aside.
4) When oil is heated, drain chips and thoroughly pat dry with paper towels. Add 1/3 of chips to oil and cook until browned and cooked through, about 4 to 5 minutes. Place onto paper towels with a slotted spoon and season immediately with salt. Return oil to 180C and repeat with remaining chips. Keep warm in oven until fish is ready.
5) For the fish: Skim oil to clean then return to stove over medium-high and bring to 180C. Pat fish dry with paper towels then cut into 1.5cm strips; set aside. In a large bowl whisk together flour, salt, cayenne, and pepper until combined. Add club soda and whisk briefly until a batter is formed (there may be a few lumps left).
6) Place all fish in batter and mix to coat thoroughly. Remove 8 pieces of fish from batter, shake off excess and fry, stirring occasionally, until golden and fish is just cooked through, about 2 to 3 minutes. (To test if fish is cooked, break open 1 piece and make sure meat is white throughout.)
Transfer fish with a slotted spoon to paper towels to drain. Return oil to 180C and repeat with remaining fish. Serve immediately with chips and mayonnaise.