In a pan large enough to take the other ingredients, melt the duck fat or butter and cook your onions. This time we want them to achieve a soft, sweet brownness (no burning—this is not a process you should rush; it will take up to 1 hour). Once achieved, add the cider and stock, bring to a simmer and cook for 30 minutes. While this is happening, roast your marrowbones in a hot oven until the marrow is loose, not flowing out of the bones. Sprinkle the bread with olive oil and toast in the oven. Season the soup to taste.