Butter bottom and sides of 2- to 2 1/2-quart souffle dish or straight-sided casserole.  Dust with crumbs.  Make 4-inch wide band of triple-thickness aluminum foil long enough to go around dish and overlap 2 inches.  Lightly butter 1 side of band and dust with crumbs.  Wrap around outside of dish, dusted side in, and fasten with tape, paper clips or string.  Collar should stand at least 2 inches above rim of dish.  Set aside.

In medium saucepan over medium-high heat, melt butter.  Stir in flour, mustard and salt, if desired.  Cook, stirring constantly, until smooth and bubbly.  Stir in milk all at once.  Cook and stir until mixture boils and is smooth and thickened.  Set aside.

In large mixing bowl, beat egg whites with cream of tartar at high speed until stiff but not dry, just until whites no longer slip when bowl is tilted.  Stir egg yolks into reserved sauce until thoroughly blended.  Stir in shrimp, clams, onions and juice.  Gently, but thoroughly, fold yolk mixture into whites.  Carefully pour into prepared dish. 

For a “top hat”, hold spoon upright and circle mixture to make ring about 1 inch from side of dish and 1 inch deep.  Bake in preheated 350° F oven until puffy, delicately browned and souffle shakes slightly when oven rack is moved gently back and forth, about 50 to 60 minutes.  Quickly, but gently, remove collar.  Serve immediately.