1. Preheat oven to moderate (350° F.). Cook shrimp, covered, in one-half cup water five minutes. Drain, reserving the broth.
2. Cream the butter with the mustard and spread on the bread. Cut into cubes. Arrange the bread cubes, cheese and shrimp in layers in a greased one-quart casserole.
3. Mix the eggs, milk, reserved shrimp broth, salt and pepper and pour over the shrimp mixture. Set in a pan of hot water and bake until a knife inserted in the center comes out clean, one hour or longer