Peel the onions and cut them into thick, approximately ½-inch slices. Separate the slices into rings.
Set up your breading station with three bowls. In one bowl place the flour, salt and pepper. Mix well to combine. In the second bowl, beat together the eggs and milk and in the third bowl, place the panko crumbs.
Prepare two large baking sheets by lining them with parchment paper. Spray the parchment lightly with cooking spray. Set aside.
Coat the onion rings by dredging them first in the flour. Remove from the flour, shake off any excess and drop into the egg and milk mixture. Remove the onion ring from the egg mixture allowing any excess to drip back into the bowl and then place the coated ring into the panko crumbs. Toss lightly in the crumbs to coat.
Place coated onion rings on the prepared baking sheet. Repeat until all onion rings are coated.
Spray the prepared onion rings lightly with cooking spray. Place the baking sheets in the oven and cook for 10 minutes. Remove from oven and flip each onion ring carefully using a spatula or tongs. Replace the sheets in the oven and cook for an additional 10 minutes or until crispy and browned.