Pre-heat the oven to 350°F.
Melt the butter in a skillet; add the apple, celery and onion. Season with salt, pepper and the bay leaf and thyme and cook until tender, 10 minutes or so. Add the parsley and deglaze with the Calvados or brandy; add the sausage and lightly brown – do not cook the sausage until dry. Stir in the stuffing; season with a little celery salt and moisten with stock, adding just enough to make the bread damp, but not wet. Stir in the chestnuts, transfer to a casserole dish and bake for 35–40 minutes.