Cut off the top of each onion and peel away the skin. Using a sharp knife, cut the onions into 1/4-inch slices (or thicker if you prefer). Separate the individual onion rings from each slice, discarding the centers. Set aside.
In a medium mixing bowl, whisk together flour, seasoned salt, and baking powder. Add in water and eggs and mix until the batter is smooth.
Heat 2-inches of oil in a large pot to 375 degrees F over medium-high heat. You can use a long stem thermometer to monitor the oil's temperature.
Dip onion slices in the batter, and shake gently to remove any excess. Then carfeully drop the onions into the hot oil. Do not overcrowd the pot.
Cook for 2 to 4 minutes per side, or until golden brown. Use tongs or a spider strainer to remove cooked onion rings to a paper towel-lined baking sheet. Repeat with remaining onions and batter.
Ser onion rings immediately. I like them with fry sauce or ketchup for dipping.