Sauté celery, mushrooms, green pepper, green onion, and 1 tablespoon parsley in 2 tablespoons butter. Stir in flour, and cook 1 additional minute. Add clams and cook until thickened, stirring constantly.
Gradually stir about one-fourth of clam mixture into egg; add egg mixture to remaining clam mixture, stirring constantly. Stir in lemon juice, salt, and garlic powder. Place 1 tablespoon of clam mixture into greased clam shells. Set clam shells aside.
Combine breadcrumbs, cheese, and melted butter; toss lightly. Sprinkle crumb mixture evenly over clam mixture. Top with 1/4 cup parsley, and sprinkle with paprika.
To bake, pour enough rock salt into a 13- x 9- x 2-inch baking dish to cover bottom (salt helps shells sit upright); arrange filled shells on salt. Bake at 400° for 10 minutes or until lightly browned.