Peel onions and trim root ends; cut off tops to create wide opening. Using spoon or melon baller, scoop out onion, leaving shell about 2 onion layers thick. Finely chop some of the scooped out onion to equal 3/4 cup (175 mL); refrigerate or freeze remaining onion for another use.

In large Dutch oven of boiling water, cook onion shells for 5 minutes; drain with tops down on paper towel.

In large nonstick skillet, cook bacon and chopped onions over medium heat until onion begins to soften. Stir in mushrooms, garlic, parsley and thyme; cook for 3 minutes Season with salt and pepper to taste.

Place onions upright in casserole just large enough to hold them snugly. Stuff onions with mushroom filling. Combine beef stock with vinegar; pour around onions. Cover with foil; bake in 375ºF (190ºC) oven for 40 minutes. Baste with pan juices; cook, uncovered, for 15 to 20 minutes more or until onions are tender.