Preheat oven to 350°. Remove and discard root ends and dark green tops of leeks. Thinly slice leeks, and rinse thoroughly under cold running water to remove grit and sand.
Rinse mushrooms, and pat dry. Remove and discard stems. Place mushrooms, upside down, on a wire rack in an aluminum foil-lined jelly-roll pan. Sprinkle with 1/2 tsp. salt; invert mushrooms.
Bake at 350° for 15 minutes.
Sauté leeks, shallots, and garlic in hot oil in a large skillet over medium heat 3 to 5 minutes or until tender. Transfer mixture to a large bowl. Stir in 1/4 cup Parmesan cheese, next 3 ingredients, and remaining 1/2 tsp. salt until well combined. Spoon 1 heaping teaspoonful leek mixture into each mushroom cap. Sprinkle with remaining 1/4 cup Parmesan cheese. Bake at 350° for 10 minutes or until golden. Garnish, if desired.