Preheat the oven to 375 degrees F.
Put the apples on a flat surface. Cut the cheeks from the apple, to remove the core. Cut into fairly thin, but not too thin, slices. Put them in a bowl and add the lemon zest and juice and the orange zest and juice. Toss to coat the apples, then add the molasses, 1/2 teaspoon salt, the cinnamon, ginger, and the nutmeg. Toss to blend.
In another bowl, combine the flour, brown sugar and remaining 3/4 teaspoon salt. Add the butter to the bowl and break it up with your fingers integrating the flour with the butter. Spread it out on a plate or small baking sheet and refrigerate until ready to use.
Butter the bottom and sides of a shallow rectangular baking dish (approximately 13 inches long and 9 inches wide) with the remaining tablespoon of butter. Add the apples and top with the butter-flour mixture. Put the dish in the center of the oven and bake until the apples are tender when pierced with the tip of a knife. The topping should be golden brown as well, about 35 to 45 minutes. Remove from the oven and set aside to cool a few minutes before serving. To serve, garnish with additional lemon zest and ice cream, if desired.
Recipe courtesy of Alex Guarnaschelli