Preheat oven to 425°. Remove papery outer skin from garlic bulbs (do not peel or separate cloves). Cut top off of each garlic bulb. Brush with 2 tablespoons oil. Top with rosemary sprigs; wrap in heavy-duty foil. Bake until softened, 30-35 minutes. Cool for 10 minutes.
Reduce oven temperature to 400°. Squeeze softened garlic into a small bowl. Stir in the cheese, melba crumbs, 1/4 cup olive oil, lemon juice, minced rosemary and lemon zest.
Remove stems from mushrooms (discard or save for another use).
Fold each prosciutto slice in half lengthwise; wrap around mushrooms and fill with cheese mixture. Place in a greased 13x9-in. baking pan; drizzle with remaining oil. Pour broth around mushrooms.
Cover and bake 20-25 minutes or until heated through. Uncover; broil 3-4 in. from the heat until topping is golden brown, 2-3 minutes.
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