How to Make It
Preheat oven to 400°F. Lightly coat a 13- x 9-inch baking dish with cooking spray.
Place onions on a work surface. Working from stem end of onion, cut in the center toward the root end, leaving 1/4 inch uncut. Cut again to form 4 quarters, leaving root end intact. Repeat with all onions, placing root sides down in baking dish. (If they fall over, that’s OK.) Coat onions with cooking spray, and drizzle with vinegar. Bake at 400°F until slightly softened, 30 to 35 minutes. Remove from oven, and let stand at room temperature.
While onions bake, toss bread cubes with 2 tablespoons oil in a large bowl; spread in an even layer on a baking sheet. Place in oven with onions, and bake until crunchy, about 10 minutes. Remove bread cubes from oven, and place in a large bowl.
Heat remaining 1 tablespoon oil in a large skillet over medium-high until shimmering. Add sausage, and cook, stirring occasionally, until beginning to brown, about 6 minutes. Stir in celery and cranberries; cook, stirring often, until celery is slightly softened, about 2 minutes. Add sausage mixture to bowl with toasted bread cubes. Stir in eggs, stock, parsley, pepper, and salt until combined. Let mixture stand at room temperature until liquid is absorbed into bread, about 10 minutes. Lightly pack about 1/2 cup stuffing into each onion. Lightly spray tops of stuffed onions with cooking spray. Bake at 400°F until stuffing is lightly golden, about 15 minutes.