Peel shrimp. Sauté the onions, but don't let them brown; set aside.
Sauté the mushrooms until they are soft.
Add the sherry and nutmeg, and simmer until the liquid is almost gone; set aside.
Heat the half and half to a simmer; set aside.
In a medium glass or ceramic dish, layer the potatoes, onions, mushrooms, shrimp, breadcrumbs, and Parmesan repeatedly until the potatoes are gone, finishing with a layer of potatoes.
Pour over the half and half, covering the top layer, and cover the dish with its lid.
Bake at 350 degrees F. for 1 hour, then test the potatoes.
If they are tender, sprinkle with breadcrumbs and Parmesan and dot with butter.
Turn heat to 400 degrees F. and cook, uncovered, until brown.