Beth’s Oyster Gratin with oyster mushrooms

This recipe is from Beth Snow, who is the computer whiz whose help was invaluable in putting this book together. She writes: “This dish was always included at my family’s holiday dinners, served alongside roast turkey and all the trimmings. I’ve adapted this recipe from the original by adding mushrooms and lemon zest.” If you can find only large, jarred oysters, cut them in half to use here.