In a large skillet, cook spinach in a small amount of boiling water 3 to 5 minutes or until just tender. Drain. Transfer spinach to small bowl.
In the same skillet, cook onion, mushrooms, and garlic in the 1 tablespoon butter or margarine until tender. Stir in oregano, salt, and pepper. Stir into spinach in bowl; set aside.
Place each chicken breast half between 2 sheets of clear plastic wrap. Pound lightly with the flat side of a meat mallet into a rectangle about 1/8 inch thick. Remove plastic wrap.
Place one-sixth of spinach mixture on each chicken breast half. Fold in bottom and sides; roll up. Secure with wooden toothpicks, if necessary.
In a small bowl, mix bread crumbs, Parmesan cheese, and paprika. Dip rolls in a mixture of beaten egg and the 1 tablespoon water. Coat with the bread crumb mixture. Place rolls, seam sides down, in a lightly greased 12x7x2-inch (2-quart rectangular) baking dish. Drizzle with the 2 tablespoons melted butter or margarine.
Bake, uncovered, in a 350 degree F oven for 20 to 25 minutes or until chicken is tender and no longer pink. Serve hot. (Or, chill the baked chicken in the refrigerator for at least 6 hours.) Makes 6 servings.