1 1/2 ounces dried porcini mushrooms
1 cup warm water
2 tablespoons olive oil
1 cup chopped onion
2 garlic cloves, minced
1/2 cup dry Marsala
1/2 cup dry white wine
1 teaspoon minced fresh rosemary
1 cup chicken stock or canned low-salt chicken broth
1 cup beef stock or canned beef broth
1 tablespoon butter, room temperature
1 tablespoon all purpose flour
Combine porcini mushrooms and 1 cup warm water in small bowl. Let stand until mushrooms soften, about 30 minutes. Remove mushrooms from liquid, squeezing excess liquid from mushrooms back into bowl; reserve liquid. Place mushrooms in another small bowl.
Heat oil in heavy large saucepan over medium-high heat. Add onion and garlic; sauté until onion browns, about 15 minutes. Add Marsala and white wine. Increase heat; boil until most liquid evaporates, about 7 minutes. Add rosemary, mushrooms and both stocks. Pour in reserved mushroom liquid, leaving any sediment behind. Boil until liquid mixture is reduced to 2 cups, about 15 minutes.
Mix butter and flour in small bowl to blend; whisk into mushroom mixture. Simmer until sauce thickens, about 2 minutes. Season with salt and pepper.