Preheat oven to 375°.
Pierce potatoes with a fork, and bake at 375° for 1 hour or until tender. Cool potatoes slightly.
Heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add leek; cook 5 minutes or until tender, stirring frequently. Add mushrooms; cook 5 minutes or until tender, stirring occasionally. Stir in sour cream, broth, cheese, salt, and pepper. Reduce heat, and cook 2 minutes, stirring constantly (do not boil). Stir in tarragon.
Split potatoes lengthwise, cutting to, but not through, other side. Spoon 3/4 cup mushroom mixture into each potato.