Position racks in top and bottom of oven. Preheat to 450F. Spray 2 baking sheets with oil. Scoop 1/3 cup all-purpose flour into a freezer bag. Whisk 1 egg with 2 tbsp milk in a bowl. Scoop 1/3 cup bread crumbs, 1/3 cup cornmeal, 1 tsp Italian-herb seasoning and 1/2 tsp salt into a second freezer bag. Slice 1 Spanish onion into rounds. Separate into rings. Shake rings in the flour, then dip each ring into the egg mixture. Shake in crumb mixture. Lay on sheets. Repeat. Spray rings with oil. Bake in oven until golden, switching sheets halfway through, 15 min.