Beat 1/2 C. chilled whipping cream in chilled bowl until stiff. Fold in 3 T. well-drained prepared horseradish and 1/2 tsp. salt.
Cook spinach as directed on package or sauté fresh in very small amount of oil until wilted; drain well. Cook and stir celery, mushrooms, and onion in oil until celery is tender. Stir in spinach, salt and pepper. Place 1 ham slice in ungreased shallow baking dish making diagonal cuts along ham's edges. (This keeps meat from curling.)
Spread ham slice with spinach mixture. Top with second ham slice making diagonal cuts along the edge. Brush second slice with butter. Cover and cook in 325 degree oven for 15 minutes. Uncover and cook 15 minutes longer. Serve with Horseradish Sauce.