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.
Add 2 inches of oil in a large pot and bring to 375 degrees F over medium-high heat. Use an oil thermometer to monitor the temperature.
Dip onion slices in the batter and gently shake to remove any excess. Then, carefully 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.
Serve onion rings immediately. I like them with fry sauce or ketchup for dipping.
Notes
Keep the onion rings warm by turning your oven to 200 degrees or the "warm" setting and placing the baking sheet with the onion rings in the oven between batches.