Combine mushrooms and 1 1/2 cups boiling water in a bowl. Cover and let stand 30 minutes or until tender. Drain in a colander over a bowl, reserving 1 cup liquid. Finely chop mushrooms.
Place each piece of pork between 2 sheets of heavy-duty plastic wrap, and pound to 1/2-inch thickness using a meat mallet or small heavy skillet. Sprinkle both sides of pork with pepper.
Combine rosemary, salt, and crushed garlic in a small bowl; mash with a fork into a paste. Heat olive oil and butter in a large nonstick skillet over medium-high heat. Add garlic paste; sauté 30 seconds. Add pork; cook for 2 minutes on each side. Remove from pan. Add diced onion to pan; sauté for 5 minutes or until tender. Add chopped mushrooms and Marsala, and cook for 1 minute. Add reserved mushroom liquid and broth; bring to a boil. Cook until reduced to 1 1/2 cups (about 10 minutes). Reduce heat to medium-low. Return pork to pan; cook 3 minutes or until thoroughly heated. Garnish with rosemary sprigs, if desired.