Combine flour, salt, and a pinch of cayenne in a medium bowl. Whisk in buttermilk, beer, egg, and lemon zest, and let stand for 20 minutes.
Preheat oven to 200 degrees. Place a baking sheet on middle rack of oven. Heat oil to 380 degrees in a large, heavy saucepan over medium-high heat. Dip a few slices of onion in batter, turning to coat. Gently drop slices into oil. Cook, turning once with a wire-mesh skimmer, until golden brown, about 2 minutes. Transfer rings to paper towels, season with salt immediately, then transfer onions to the baking sheet to keep warm. Repeat with remaining onions. (Adjust heat as necessary to keep oil at a steady temperature.) Serve with lemon wedges for squeezing.