Preheat oven to 425°. Cut a thin slice from bottom (rounded end) of each onion, forming a base for onions to stand. Cut 1/2 inch from sprout end of each onion, and discard. Peel onions. Remove and reserve center of each onion, leaving 2 layers of onion as a thick shell. Finely chop 1 cup reserved onion centers. Reserve remaining onion centers for another use, if desired.
Place hollowed onions in a 2-qt. baking dish, and add 1 cup broth. Cut 2 Tbsp. butter into 8 pieces, and place 1 piece in each onion. Cover with aluminum foil, and bake at 425° for 25 to 30 minutes or until shells are tender. Remove from oven to a wire rack, and cool 20 minutes.
Pour liquid from onions into a measuring cup, and add broth (about 1/4 cup) to equal 1 cup. Reduce oven temperature to 350°.
Melt 4 Tbsp. butter in a large skillet over medium-high heat; add celery and 1 cup chopped onion, and sauté 5 minutes or until tender. Stir in parsley and sage, and cook, stirring constantly, 1 minute or until fragrant. Stir in breadcrumbs, salt, pepper, and 1 cup reserved broth mixture. Remove from heat.
Fill hollowed onions with stuffing, and place in a shallow 9-inch baking dish or pie plate. Add remaining 1/4 cup broth to pan. Bake at 350° for 30 to 35 minutes or until stuffing is golden brown and thoroughly heated. Transfer to a serving platter. Stir remaining 2 Tbsp. butter into cooking liquid, and spoon over onions.