Peel skin from onion and cut off top 1/4 inch. Leave root end on to hold onion together. Cut through the onion from the top down to the bottom, stopping 1/2 to 1/4 inch short of bottom.(leave bottom intact), forming 16 wedges.
Place on a 12 inch square sheet of foil. Fold up foil around the onion and place in an oven safe baking dish or pie plate. Bake for 20 minutes.
Meanwhile combine the butter and mustard in a small bowl and mix well.
Remove the onion from the oven and open the foil. Fold it down to make a 1 inch high rim around the onion; crimp edges to seal. With a pastry brush, coat the onion with the butter mixture, then sprinkle liberally with bread crumbs and paprika.
Return onion to the oven and bake an additional 15 to 20 minutes, or until crispy tender. Pour the ranch dressing into a small bowl. Place the cooked onion on a serving plate and set the dressing bowl in the center or alongside.
TIP: Any favorite dressing that goes well with onions can be used for dipping sauce.