Preheat the oven to 350 degrees F. Place the tart shell in the oven and bake for 10 minutes. Remove and cool. Set aside. In a mixing bowl, combine the creme fraiche, cheese and egg. Mix well. Season with salt and pepper, set aside. Slice the tomatoes 1/4-inch thick. Season both sides of the tomatoes with salt and pepper. Line the bottom of the tart shell with the tomatoes. Lay the onions over the tomatoes. Season both sides of the potatoes with salt and pepper. Fan the potatoes evenly over the onions. Pour the cheese mixture evenly over the potatoes. Spread evenly. Bake for 10 to 12 minutes, or until golden brown. Remove from the oven and cool for 5 minutes before serving. Slice the tart into 8 servings. Garnish with parsley.