Cut off the top quarter from:
Trim off any roots, but leave the root end intact. Peel off the skin. Use a sharp paring knife to carefully cut out a cone-shaped section from the center of each onion and use a spoon to dig out all but ¼ to ½ inch of the onion. There should be 2 to 3 layers of onion left. Reserve the onion bits and coarsely chop half of them. Reserve the remaining onion for another use or save in a stock bag. Set the onions in a steamer basket above a few inches of water. Steam until the onions are tender enough to be pierced by a skewer but still feel slightly resistant, about 15 minutes. Remove the onions and let stand until cool enough to handle.
Preheat the oven to 375°F. Butter a baking dish just large enough to hold the onions and arrange the onions in the dish.
Crumble into a medium skillet and brown well over medium heat:
Without draining off the fat, add the chopped onion bits and cook, stirring, until golden and very soft, about 10 minutes. Meanwhile, squeeze dry and finely chop:
Add the spinach to the sausage mixture, reduce the heat to medium-low, and cook for 5 minutes. Pour in:
Cook for about 1 minute more; the mixture should be quite thick. Remove from the heat. Stir in:
Pile the sausage stuffing into the onions. Sprinkle with:
Bake until lightly browned, 25 to 30 minutes. Let stand for a few minutes before serving.