Peel and core the pears and remove their stems; then, cut them in half. Bring 1 quart water to a boil in a large saucepan. Add the lemon peel. Reduce the heat and poach the pears in batches in a single layer just until tender. Remove them with a slotted spoon, drain, and set aside.
Cream the butter with an electric mixer until light. Separate the eggs and add the yolks to the butter. Add the flour, baking powder, milk, and salt.
Preheat the oven to 400 degrees F and grease a 2-quart gratin dish.
Beat the egg whites with salt until they stand in stiff peaks and fold them into the dough.
Line the bottom and the sides of the gratin dish with the ham. Place the pears cut side down in the dish in a single layer and pour the dough over them. Bake for 30 to 35 minutes, or until golden and a knife comes out clean. Serve warm.