Melt 2 tablespoons butter with 2 tablespoons oil in large skillet over medium-high heat. Add fresh mushrooms; sauté until browned, about 6 minutes. Add porcini and garlic; stir 1 minute. Remove from heat. Combine flour, remaining 2 tablespoons butter, and remaining 2 1/2 teaspoons crushed rosemary in small bowl; mash with fork to smooth paste. DO AHEAD Porcini soaking liquid, mushroom mixture, and flour-butter mixture can be made 1 day ahead. Cover separately; chill.