1. Prepare Buttery Flaky Pastry.
2. Preheat oven 350°. Bring potatoes, 1 tsp. salt, and water to cover to a boil in a large skillet over medium-high heat; cook 5 to 7 minutes or until tender. Drain potatoes, and pat dry with paper towels. Cool 10 minutes.
3. Cook onions in boiling salted water to cover in a small saucepan 2 to 3 minutes or until tender. Drain well, and press between paper towels.
4. Stir together parsley and next 2 ingredients in a small bowl.
5. Whisk together half-and-half, next 4 ingredients, half of parsley mixture, and remaining 1/2 tsp. salt.
6. Spread half of potatoes in bottom of pastry crust. Top with half of Gruyère cheese, half of cooked onions, and half of egg mixture; repeat layers once. Sprinkle with Parmesan cheese.
7. Bake at 350° on an aluminum foil-lined baking sheet 40 to 45 minutes or until center is set. Let stand 10 minutes. Sprinkle with remaining parsley mixture. Serve warm or at room temperature.