Reduce 4 cups Butter Sauce Base to 3 cups in saucepan over medium-low heat (about 10 minutes).
Sauté mushrooms and shallots in unsalted butter for 3 minutes over medium heat. Add tomatoes and rosemary. Pour butter sauce reduction into sauté mixture. Let simmer over low heat for 2 minutes to blend flavors. Season.
Serve over grilled beef, chicken or pasta.