Special equipment: a mandolin; a deep-frying thermometer
Mix the flour, baking powder, seasoned salt and pepper in a large bowl. In a pitcher, mix together the beer and eggs. Pour the liquid into the dry ingredients and mix.
In a large heavy-bottomed pot, heat enough vegetable oil to come halfway up the sides of the pot until a deep-frying thermometer inserted in the oil registers 375 degrees F.
In batches, dip the onion strings into the batter, then plunge them into the hot oil and cook until golden, 2 to 3 minutes. Drain on a paper towel and repeat with the rest of the onions. Sprinkle with seasoned salt.