Preheat the oven to 400 degrees.
Place the marrow bones in a shallow baking dish. Peel and quarter one of the onions and add it to the pan.
Roast the bones and onions for 20-25 minutes or until nicely browned, turning once.
Remove the bones and onions from the pan and place in a stock pot with 1/2 tsp salt. Pour a quarter cup of water into the roasting pan and gently heat on the stove top, scraping to remove stuck on bits of meat. Add pan juice to the stock pot.
Add 4 cups cold water to the stock pot and bring to a simmer. Cover the pot and simmer gently, on low, for 2 hours.
Remove onions and bones from the stock pot. Scoop the marrow from 2 of the bones and add it to the simmering stock, stirring until it partially dissolves.
Finely chop the second onion and add it, along with the celery, stuffing and turkey to the pot. Bring back to a simmer and add salt to taste. (The stuffing may add enough seasoning.) Cover the pot and simmer for 1 1/2-2 hours.
Stir in the frozen spinach and bring soup back to a simmer. Season with salt and pepper to taste. Serve immediately.