In a plastic zipper bag, combine onion rounds, 2 cups of beer, vinegar, salt, pepper and granulated garlic. Close bag, sealing tightly, and shake vigorously to mix ingredients. Refrigerate at least 30 minutes or up to 2 hours. NOTE: Do not refrigerate onion rounds longer than 2 hours or they will become soft and too saturated to crisp properly. When ready to cook, preheat oven to 200°F and adjust oven rack to middle position. In a Dutch oven, heat oil to 350 degrees F over medium-high heat. In a large bowl, combine flour, cornstarch and baking powder then season to taste using salt, pepper and granulated garlic. Slowly whisk in 3/4 cup of beer until just combined. Whisk in remaining beer as needed, 1 tablespoon at a time, until batter falls from whisk in a steady stream and leaves a faint trail across surface of batter. Set batter aside. Line a baking sheet with paper towels and set aside. Remove onions from refrigerator and drain, discarding liquid. Pat onion rounds dry with paper towels and separate into individual rings. Transfer one-third of rings to batter, turning to coat well. Carefully add battered rings to oil, one at a time. Fry approximately 5 minutes or until golden brown and crisp, flipping once. Using tongs, remove rings from oil and set on prepared baking sheet to drain. Season to taste with salt and pepper. Place in oven to keep warm until all onions are fried. Return oil to 350 degrees F and repeat with remaining onion rings. Serve immediately.