Heat butter in a large skillet over medium heat. Add leeks; cook and stir until tender but not browned, about 8 minutes. Lower heat to medium-low and add mushroom sage oil and mushrooms. Cook and stir until mushrooms reduce by half, about 15 minutes. Stir in garlic, and let cook until soft but not brown, 2-3 minutes. Deglaze with lemon balsamic, then add the Parmesan cheese, and rosemary. Season to taste with salt and pepper. Stir in crème fraiche. Remove from heat and allow mushroom mixture to cool. It will thicken as it cools.