In a small bowl whisk the apple juice together with the cornstarch. Set aside. Toss the apples and lemon juice together in a large bowl. Melt the butter in a large skillet over medium heat. When the butter is melted and starts to sizzle add the apples, brown sugar, cinnamon, and salt. Stir gently until well combined. Cook until the sugar melts and the mixture starts to bubble. Turn the heat down to medium-low and simmer until the apples soften a bit, about 2-3 minutes. Stir in the cornstarch mixture. Continue cooking until the sauce thickens, about 2 minutes. Transfer the contents of the pan to a bowl and set aside to cool to room temperature, about 20 minutes.