Preheat the over to 425 degrees. Lightly grease a large baking or roasting pan. Peel the Walla Walla Sweet Onions, cut the bottom level, but leave the core intact. To make the flower design, begin as if you are going to cut the onion in half, from top to bottom, but stop cutting 1/2 inch from the core. Cut the onion this way 5 times to form ten sections or "petals." Carefully loosen the petals slightly by rapping the onion lightly on the work surface, and loosening them with your fingers. Remember that the flower will bloom more as it cooks.
Place flour, salt, pepper, oregano, paprika and mustard in a small paper bag. Spray the onions with cooking spray to lightly coat each petal. Put one of the onions in the bag and gently shake to coat. Remove the onion and pat off the excess flour mixture. Place in a prepared pan and repeat with the other onion. Spray the coated onions lightly again with cooking spray to aid in browning.
Bake the onions in the preheated oven for 35 to 40 minutes, until tender. Serve the onion flower hot, with your favorite barbecue sauce as a dip, or as an accompaniment with pork loin, or grilled steak.