Peel and cut onion into ¼-inch slices. Separate each slice and throw out the center. Set aside.
In a large bowl, whisk together flour, baking powder, and season salt. Add in water and eggs, mix until smooth.
Heat a large pot over medium heat. Pour oil to cover at least 2 inches, using a thermometer, wait until the oil is 375°F .
Dip each onion slice into the batter, shake off excess, then drop into hot oil. Work in small batches.
Cook for 3 minutes each side. Use tongs to remove and strain remaining oil. Pat onion rings with a paper towel. Repeat for remaining rings.
Serve with ketchup.
