Cut off enough of the top of the garlic head to expose almost all the cloves. Wipe some butter over the exposed area, and place it cut side down in a small baking dish in a 250-degree oven. Bake it for an hour and a half to two hours, until the garlic is soft and spreadable.
Dissolve the salt into six cups of water in a saucepan. Add the onions, and brig the pot to a light boil.
Cut an X in the flat side of each chestnut, and add them to the boiling water. Boil uncovered for 40 minutes.
Remove the chestnuts. When cool enough to handle, remove the chestnut skins. Discard the onion and boiling liquid.
Squeeze out all the garlic cloves and combine them with the chestnuts in a food processor. Chop roughly.
In a large saucepan, melt the butter and saute the celery and the crushed red pepper until the celery is soft. Add the sherry and cook for a minute to evaporate the alcohol. Add chestnut-garlic mixture, the stock, bay leaves, parsley, and nutmeg. Cook for about 15 minutes.
Strain the soup and puree the solids. (If you have a food mill or a Mouli, use it to make the puree.) Return the puree to the pot along with the half-and-half. Simmer for about 30 minutes. Add salt and pepper to taste, and serve with croutons.