In a large skillet, saute mushrooms in butter for 2-3 minutes. Stir in flour until blended; cook 2-3 minutes longer or until liquid is absorbed. Gradually stir in broth and cream. Bring to a boil. Remove from the heat; stir in the sherry or additional broth, mustard and lemon juice. Cool for 15 minutes.
Grease and line a 15-in.x 10-in. x 1-in. baking pan with parchment paper; grease the paper. Sprinkle with bread crumbs; set aside. In a large bowl, combine spinach, butter, salt, pepper, nutmeg and egg yolks. In a small mixing bowl, beat egg whites on high speed until stiff peaks form. Gradually fold into spinach mixture. Gently spoon over bread crumbs; press down lightly. Sprinkle with Parmesan cheese.
Bake at 350 degrees F for 12-15 minutes or until center springs back when lightly touched. Cover with a piece of greased foil; immediately invert pan onto foil. Gently peel away parchment paper. Spread 1 cup mushroom sauce over spinach mixture to within 1 in. of edges. Roll up jelly-roll style, starting with a short side and peeling foil away while rolling. Cut into slices. Reheat remaining mushroom sauce; serve with spinach spirals.