A great buffet dish. It should be made well ahead so the flavors have time to meld.
Butter a 2-quart casserole dish and layer with half the shrimp, half the bread, and half the cheese. Repeat for a second layer of each, ending with the cheese. Pour the melted butter over the cheese. Beat together the eggs, mustard, and milk. Pour over the casserole. Cover and refrigerate for at least 3 hours, preferably overnight.
Preheat the oven to 350 degrees F. Bake, covered, for 1 hour. Serve warm.