Place unpeeled onions in a bowl and cover with very hot water. Set aside for 15 minutes to soften skins. Drain onions, then peel and pat dry. Heat a large skillet over high heat and add a - count of olive oil. Add the onions and saute until golden brown all over, about 10 minutes. Add the vinegar and honey, reduce heat to medium, and cook until mixture has reduced to a syrup and onions are soft, about 20 minutes.
Preheat oven to 400°F.
Heat a large pot over medium heat. Drizzle with a 2-count of olive oil, add butter, and stir around until it melts. Add onions and garlic and saute until soft, about 5 minutes. Add spinach in batches, pushing down into pan with a wooden spoon to help it wilt and adding more spinach to the pot as soon as room is available. Cook spinach until it is dry, then reduce heat and add cream and nutmeg. Stir and cook for 10 minutes, Season with salt and pepper.
Pour the spinach into a baking dish and top with onions and panko. Bake until bread crumbs are golden brown, about 15 minutes. Serve hot.6.