Preheat the oven to 400°.
Combine the bread, chopped parsley, lemon peel, sage, 1 tablespoon of salt and the pepper in a large deep bowl and toss with a spoon until well mixed.
In a heavy 10- to 12-inch skillet, melt the butter bits over moderate heat. When the foam begins to subside, add the chopped onions. Stirring frequently, cook for about 5 minutes until they are soft and translucent but not brown.
Stir in the celery and cook for a minute or so; then, with a rubber spatula, scrape the entire contents of the skillet into the bread mixture.
Add the oysters and egg and stir the ingredients gently but thoroughly together. Taste the oyster stuffing for seasoning.
Bake until hot throughout and crusty on top.