Discard any clams that remain open when tapped with fingers. Chop clams and shrimp into 1/4X1/4-inch pieces. Set aside.

Preheat oven to 350°F. Grease 4 (6-inch) casserole dishes. Slice 4 mushroom caps. Melt 1 tablespoon butter in large skillet over medium heat. Add sliced mushrooms; cook and stir until tender. Set aside.

Melt remaining 7 tablespoons butter in same skillet over medium heat. Add scallops, shrimp, diced celery and onion. Cook and stir over medium heat about 2 minutes or until shellfish is opaque. Add clams with reserved juice to skillet. Bring to a boil. Reduce heat to low. Simmer 1 minute; add white wine. Remove from heat. Fold in bread crumbs.

Place 6 mushroom caps in each prepared casserole dish. Using small spoon, fill mushroom caps with stuffing mixture. Bake 10 minutes.

Remove from oven. Place sliced mushrooms on top of each casserole. Top each with 1/4 cup of Hollandaise Sauce; return to oven just to heat through. Serve hot. Garnish, if desired.

Place egg yolks and water in top of double boiler. Whisk until well combined.

Reserve 2 tablespoons butter. Melt remaining butter in small saucepan over low heat until just melted.

Add reserved 2 tablespoons butter to egg yolk mixture; place double boiler top over hot, not boiling, water. Whisk constantly until egg yolks thicken slightly.

Remove egg yolk mixture from heat. Gradually add melted butter to egg yolk mixture, whisking constantly.

Whisk in remaining ingredients. Use sauce immediately.