Rinse fillets, and pat dry; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Combine 3 tablespoons flour and paprika; dredge fillets in flour mixture.
Saute one-third of fillets in 1 tablespoon butter in a skillet over medium heat 1 minute on each side or until opaque. Remove from skillet. Repeat procedure with remaining fillets and 2 tablespoons butter.
Saute spinach in 2 tablespoons butter in a skillet over medium heat 1 minute.
Melt 1/4 cup butter in a heavy saucepan over medium heat; add remaining 1/4 cup plus 1 tablespoon flour. Cook 1 minute, stirring constantly with a wire whisk. Add half-and-half. Cook, stirring constantly, until thickened. Add 1/2 cup Swiss cheese, lemon juice, and remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Stir 3/4 cup cheese sauce into spinach mixture. Spread in a greased 11- x 7- x 1 1/2-inch baking dish. Arrange fillets over spinach; pour remaining sauce over fillets.
Combine remaining 1/2 cup Swiss cheese, breadcrumbs, Parmesan cheese, parsley, and remaining 1 tablespoon melted butter. Sprinkle over casserole. Bake at 375° for 15 minutes.