Cut the top off the heads of garlic,and place on a sheet of aluminum foil. Drizzle with olive oil and season with salt and pepper. Place the 2 sprigs of rosemary on top and wrap the foil around tightly.Roast in the oven for 1 hour.
Squeeze the pulp from the garlic heads into a bowl. Mix in 1/4 cup olive oil, chopped rosemary, lemon zest, juice from half the lemon, feta cheese, and garlic toast crumbs. Sprinkle with salt and pepper.
Note:Stuffing can be made a day ahead. However, do not mix toast crumbs into the mixture until ready to stuff.
MUSHROOMS: Preheat oven to 400 degrees.
Fill mushroom caps with stuffing, and wrap with a prosciutto slice. Place in a greased baking pan, drizzle with olive oil, and season with salt and pepper.
Put chicken stock in the dish to just cover the bottom. Cover with foil and bake 15 minutes.
Spoon pan juices over the mushrooms, re-cover and bake 10-15 minutes more. Baste again, then broil uncovered for 2-3 minutes until prosciutto is brown and crisp.