Cut off about a quarter from the top of each onion and carefully empty it out, using a vegetable cutting spoon (melon-ball cutter) and leaving a double-walled thickness at the sides and an equivalent thickness at the bottom. Chop the tops and inside finely and stew them gently along with the slivered bacon in 1 tablespoon butter for 15 or 20 minutes, stirring regularly. The bacon should sweat, remaining limp, and the onions should be only soft and yellowed. Stir in the parsley and the breadcrumbs and remove from the heat a minute or so later.
Toss the chopped or juliènned mushrooms in 1 tablespoon butter, salted, over a high flame until their liquid is evaporated and combine with the other mixture. Season to taste and gently, but firmly, pack the stuffing into the onion cases, mounding it with the hollow of a tablespoon. Arrange them in a casserole or fairly deep oven dish just large enough to hold them, pour the stock into the bottom, bring to a boil on top of the stove, and cook, covered, in a 375° to 400° oven, basting regularly after the first quarter of an hour, for about 1 hour. The onion cases should be meltingly tender and, to be served out intact, a certain amount of care must be taken.