Cook the pappardelle until al dente. Meanwhile, heat the butter over medium heat. Add the diced garlic and saute until garlic is brown. Add the reserved pasta water and stir well to combine. Place the cooked pasta and preserved lemon and lemon-salt mixture and heat through for another minute, stirring well. Add the spinach and turn off the heat. Let the spinach wilt slightly, stirring into the pasta. Remove the pan from heat, and squeeze lemon juice over pasta to taste, and sprinkle in red pepper flakes if using. To serve, divide between plates and pour any pan liquid over the pasta.