Cut onion crosswise into ¼-inch thick rings and separate rings. Dust onion rings in dry tempura batter mix; set aside. Combine remaining tempura batter mix with baking powder and table salt in medium bowl; mix in eggs and milk. Dip onion rings into egg mixture, letting excess mixture drip off, and then coat thoroughly with panko. Working in batches, deep fry onion rings in hot oil preheated to 365°F, 1 to 2 minutes on each side until golden brown. Drain onion rings on wire rack with paper towels underneath and sprinkle with seasoned salt.