Tempura Onion Rings

2 Large onions, preferably vidalia, cut into 1/4-inch-thick rings
Vegetable oil, for frying
Ice
1-1/4 cups flour
1/2 cup cornstarch
1 tablespoon baking powder
1 teaspoon salt
1-1/4 cups ice water
Ketchup or ranch dressing, for dipping

1. In a large bowl of cold water, soak the onions for 10 to 15 minutes. Drain on paper towels.
2. Meanwhile, in a large, deep pot or skillet, heat enough oil to reach a depth of 2 inches over medium-high heat until it registers 360 degrees on a deep-frying thermometer. Fill a large bowl halfway with ice. In a medium bowl, whisk together the flour, cornstarch baking powder and salt. Add the ice water and stir until a thin batter forms. Place the batter bowl inside the bowl of ice.
3. Using tongs, coat the onion rings with the batter, letting the excess drip off. Working in batches, drop the rings into the oil and fry until light golden, 2-1/2 to 3 minutes. Using the tongs, transfer the rings to paper towels to drain; return the oil to 360 degrees between batches. Serve the onion rings with the ketchup or ranch dressing