Cut a 1/2-inch-thick slice from tops of onions, discarding tops, and trim just enough from bottoms for onions to stand upright. Scoop out all but outer 2 or 3 layers from each using a small ice cream scoop or spoon (don't worry if you make a hole in the bottom), reserving scooped-out onion and onion shells separately.
Coarsely chop enough scooped-out onion to measure 3 cups.
Cook bacon in 2 batches in a 12-inch heavy skillet over moderate heat, stirring, until crisp, about 10 minutes, then transfer with a slotted spoon to paper towels to drain, reserving about 1/3 cup fat in skillet.
Add chopped onion, celery, salt, and pepper to skillet and sauté over moderately high heat, stirring, until vegetables are softened, about 5 minutes.
Add garlic and sauté, stirring, 1 minute.
Transfer mixture to a large bowl and stir in spinach, bread, cashews, butter, 1 cup stock, and bacon, then cool completely.
Preheat oven to 425°F.
Arrange onion shells, open sides up, in a 13- by 9- by 2-inch baking pan, then add 1/2cup water and cover pan tightly with foil. Roast onions in middle of oven until tender but not falling apart, 25 to 30 minutes.
Reduce oven temperature to 350°F.
Transfer shells to a work surface and pour off water in pan. Fill shells with stuffing, mounding it, and return to pan. Reserve 5 to 7 cups stuffing for turkey cavity, then put remaining stuffing in a buttered shallow 3 1/2-quart baking dish and drizzle with remaining 1/4 cup stock.
Bake stuffed onions and stuffing in dish in middle of oven, uncovered, until heated through, about 25 minutes.
Stuffing can be prepared and onions can be stuffed 1 day ahead and chilled, covered. Bring stuffed onions to room temperature before baking.
