Cut off the tops of 2 large sweet onions 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 1 cup all-purpose flour, 1 1/2 teaspoons seasoned salt, and 3/4 teaspoon baking powder. Add in 1 cup cold water and 2 large eggs. Mix until the batter is smooth. It should be thin enough to drip back into the bowl in ribbons.
Heat 2 inches of vegetable oil in a large pot to 375 degrees F over medium-high heat. Use an instant-read thermometer to monitor the oil's temperature.
Pat the onion rings dry with paper towels if needed to remove any excess moisture. Dip the onion slices in the batter with a fork, and let any excess batter drip back into the bowl.
Carefully lower 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 to remove cooked onion rings to a paper towel-lined baking sheet. Repeat with the remaining onion slices and onion ring batter. Serve immediately with your favorite dipping sauce.
Notes
Store leftovers in an airtight container in the refrigerator for up to 3 days.
To reheat, cook in a 375°F oven for about 8 to 10 minutes or air fry at 375°F for about 4 to 6 minutes.