Separate onion slices into rings. In a large mixing bowl, toss rings with 1 cup flour; let stand for 30 min. In an extra-large mixing bowl, whisk together remaining flour, cornstarch, salt, sugar, garlic powder, onion powder and cayenne. In a large glass measuring cup, combine egg, water and beer; lightly whisk until combined. Add wet mixture to the dry mixture; whisk until homogeneous. In a wide, deep pot over medium-high heat, warm 2" of oil until it registers 350°. Working in batches, dip onion rings into batter, tapping each one on the side of the bowl after coating to prevent excess batter from dripping into the oil when cooked. Add rings to the pot a few at a time in a single layer; cook, turning once, until golden (about 3 min.). Transfer cooked onion rings to 1 of 2 sheet pans set in a preheated 250° oven to keep them warm while the remaining onion rings are cooked. Serve immediately after the last batch has been fried.
- When Minnesota Grown onions are not in season, other selections may be substituted in equal amounts.