Preheat the oven to 375. Trim a bit off the root end of the onions so they will stand flat. Do not peel them. Wrap them tightly in foil. They will probably release juices to stand them upright. Bake until tender, about 30-40 minutes. Remove from the oven and let stand while you make the bread crumb stuffing.
Heat the butter in a saucepan over medium high heat. Add the garlic and bread crumbs and cook until lightly browned, stirring frequently. Set aside.
I used frozen spinach for this, so just thawed a box. But if you're using fresh spinach, cook in boiling water for 3-4 minutes or steam if you'd rather. Drain the spinach, then add to a bowl of ice water to stop them from cooking more. When cool, drain and squeeze dry, then chop.
Mix the spinach and bread crumbs and herbs and cheese together.
Turn the oven up to 425. Oil a baking dish to hold the onions.
Remove the foil and when you can handle the onions, carefully peel them. Remove the insides so you are left with 2-3 layers of onion intact. If you get a hole in the bottom, lay one of the pieces you removed across it to hold in the stuffing.
Put the onions into the oiled baking dish. Spoon the bread crumb and spinach mixture into the onions, filling about 1/2 to 3/4 full. If you have extra, scatter it around the bottom. Drizzle the olive oil across the onions. Bake about 20-30 more minutes, until the onions are rich colored and the stuffing is crisp on top.