POSITION racks in top and bottom of oven. Preheat to 450F. Spray 2 baking sheets with oil.
SCOOP flour into a large resealable plastic bag. Whisk egg with milk in a bowl. Scoop bread crumbs, cornmeal, seasoning and salt into another large resealable bag.
SLICE onion into 1/2-in. rounds. Separate into rings. Add half the rings to flour mixture and shake to coat. Working quickly, dip each ring into egg mixture, then shake in crumb mixture until coated. Arrange rings on prepared sheets, then spray with oil.
BAKE in centre of oven until golden, switching sheets halfway through, about 15 min.