Heat butter in pot on medium heat. Separate stem from top of mushrooms (dice stems, save tops for stuffing). Cook about 8-10 minutes, stirring occasionally. Add garlic, cook for 2-3 minutes. Shut off heat, chill. Add spinach, cream cheese, prosciutto, vinegar and parmesan. Fold together with hands (use gloves, it's easier). Stuff mushrooms, top with bread crumbs, parmesan and a little melted butter.
Bake 400 degrees for 20-30 minutes or until golden brown and soft. Serve with hollandaise drizzled on top. Enjoy!