In the same skillet, melt 1 more tablespoon of butter, then add the shallot and garlic, and sauté until fragrant, about 30 seconds. Add the mushrooms, ½ teaspoon of salt, and ¼ teaspoon of pepper, and sauté until the mushrooms are tender and the juices evaporate, about 3 minutes. Add the Marsala and rosemary sprig, and simmer until the Marsala reduces by half, about 2 minutes. Add the broth and simmer until reduced by half, about 4 minutes.