Heat oven to 400 degrees. Bring a large pot of salted water to boil. Add onions and boil until tender, but not falling apart, 25 to 30 minutes. While onions are cooking, in a large skillet, heat the vegetable oil over medium heat, add the ground meat and let cook, breaking the meat apart with a spoon until 75 percent cooked. Remove meat to a plate and discard all but 1 tablespoon of the rendered fat and oil. Return the pan to the heat. Add the garlic and stir until golden, about 3 minutes, add the meat back to the pan. Add the thyme, cream, and nutmeg, and cook 3 to 5 minutes, or until most of the liquid has been absorbed. Add the spinach, wilt, and season to taste with salt and pepper. Remove from heat. Remove the onions from the pot with a slotted spoon and, when cool enough to handle, cut off top inch of each with a sharp knife and finely chop the tops. Trim root end of each onion just enough so the onions can stand without rolling over. Scoop out all but the 4 outer layers from the center of each onion (keeping outside intact) and finely chop, adding to rest of chopped onion. Transfer hollowed onions to an oiled 8-inch square baking dish. To the chicken and spinach stuffing, add the lemon zest, eggs and bread crumbs, and feta cheese. Season with salt and pepper. Divide the mixture evenly among the scooped-out onions. Drizzle the tops with olive oil and bake for 30 minutes. Serve hot or at room temperature.