Preheat deep-fryer to 375ºF (190ºC) and oven to 250ºF (120ºC).
In bowl, whisk flour, 1 1/2 teaspoons (7 mL) salt and cayenne. Whisk in beer until smooth. Let stand for 15 minutes.
Meanwhile, peel onion. Cut into 1/2-inch (1 cm) thick slices. Separate into rings.
In deep-fryer or deep heavy-bottomed pot, pour in enough oil to come at least 2 inches
(5 cm) up side. Heat to 375ºF (190ºC) or until a 1-inch (2.5 cm) cube of bread turns golden in 20 seconds. Using fork, dip 4 or 5 onion rings into batter; cook in oil for about 1 minute or until golden brown, turning halfway through. Using slotted spoon, transfer to paper towel-lined baking sheet. Keep warm in 250ºF (120ºC) for up to 1 hour. Repeat with remaining onions.
Meanwhile, cut fillets into serving-size pieces. Dredge fish in flour; shake off excess. Dip fish into batter and fry in oil, turning once, until golden brown, about 7 minutes. Remove with slotted spoon; drain on paper towel-lined baking sheet.
Sprinkle with salt to taste.
Serve with lemon wedges and tartar sauce.