Whisk together flour, cornstarch, cayenne pepper, garlic powder, and seasoning salt in a bowl.

In a separate bowl, whisk flat beer and eggs until combined.

Pour the wet ingredients into the dry ingredients and mix well to form a smooth batter.

Add the onion slivers to the batter and stir to coat evenly.

Heat oil in a deep fryer or large pot to 350°F (175°C).

Using heat-resistant metal tongs, carefully drop small stacks of onion batter into the hot oil.

Fry for 3 minutes, or until golden brown and crispy.

Remove onion stacks with tongs and place on a paper towel-lined plate to drain excess oil.

Serve immediately.