Begin heating oil in a deep fryer over moderately high heat. Separate onion slices into rings. Sift flour, salt, and baking powder together in a bowl. Slowly add milk, and beat until bubbles appear on surface of batter. When oil reaches 375 F dip onion rings, 4 or 5 at a time, into batter, allowing excess to drain off, and then drop into oil. Fry 1 minute until golden brown; turn rings to brown evenly if they don’t flip on their own. Remove to a paper towel lined baking dish and place uncovered in a 250 F oven while you fry the rest.