1. Cut the onions into 1/4-inch (1/2 cm) slices and separate into rings. 2. In a large bowl stir flour, cornstarch, baking powder, cayenne pepper, garlic powder, salt and pepper. Gradually add water and whisk until smooth and thick. 3. Dip onion rings in the batter, then coat with breadcrumbs. Repeat with all the rings. 4. Heat oil to 350-360F (170-180C). 5. Fry onion ring in batches until golden and crisp, about 1-2 minutes. Transfer to a paper towel to drain. Serve hot.