In a large bowl, whisk together flour, cornstarch, garlic powder, cayenne, paprika, baking powder, and salt and pepper, to taste.
Reserve about ¾ cup of the flour mixture (this is used to dust onion). Set aside until ready to dip the onion.
Pour oil into a cast iron or skillet, enough for deep frying. Preheat oil to about 375 Degrees F.
While the oil is preheating, add eggs, beer to the bowl of the flour mixture, and whisk until incorporated and the batter is smooth.
Then dip the onion ring in the dry flour mixture first, shake off any excess flour and dip into the beer batter mixture until it’s completely covered. Remove the onion ring and Let the excess batter drip off.
Carefully lower onion ring into the cast iron, fry a few rings at a time, do not over crowd the pan. Flip over at least once for even browning.
Fry until brown and crispy, about 2-3 minutes.
Remove from the fryer with a slotted spoon place on a cookie or wire rack to ensure crispness. Repeat with the remaining onion rings. Serve immediately with lemon wedges and tartar sauce.