Whisk flour, baking powder, 1/2 tsp. salt, pepper, egg, and beer in a large bowl.
Preheat oven to 200°. In a 6- to 8-qt. pot, heat 2 in. of oil to 375° over medium-high heat. Add onion to batter, turn to coat well, then put several onion rings in pot (do not crowd). Cook, turning often, until golden brown, about 6 minutes. Transfer to a rack set over a sheet pan, sprinkle with salt, and put in oven to keep warm. Repeat with remaining onions.
Allow oil to return to 375° and skim to remove any batter bits. Add fish to batter and turn to coat well. Cook fish in hot oil, turning and holding down as needed, until deep golden brown, about 7 minutes. Drain on rack and sprinkle with salt.
Add lemon slices and parsley to oil and cook until crisp, about 1 minute.
Pile fish and onion rings on a platter and garnish with fried lemon slices and parsley.
Note: Nutritional analysis is per serving.