Directions

1. Heat the oven to 350 degrees.
2. Spray a 1 1/2-quart casserole with cooking spray.
3. In a 10-inch nonstick skillet, cook the mushrooms in 2 teaspoons olive oil and 1 teaspoon butter over medium heat for 3 to 5 minutes or until browned, stirring frequently to brown all sides.
4. Remove from the skillet. In the same skillet, cook the leek and garlic in the remaining 2 teaspoons of olive oil for 3 to 5 minutes or until tender.
5. Stir in the flour, thyme, salt and pepper until mixed. Gradually stir in the milk until smooth.
6. Bring to a boil and cook for 1 minute.
7. Stir in 1/3 cup Parmesan cheese.
8. Place the potatoes in the casserole dish. Fold in the white sauce mixture, sour cream, and all but 1/4 cup of the mushrooms.
9. Sprinkle the casserole with the remaining 1 tablespoon Parmesan cheese and arrange the reserved mushrooms on top.
10. Cover and bake for 45 minutes. Uncover and bake for 20 to 25 minutes or until bubbly and the potatoes are fork-tender.