Put the apples, red onion, sugar, cinnamon, apple cider vinegar, 1 Tbsp. orange zest and pepper into a medium-large saucepan. Place over medium heat and gently stir together until well combined. Once this mixture begins to boil, turn down the heat and let it simmer for 45 minutes. Be sure to stir occasionally. The apples and onions will become soft and the liquid will start to thicken and create a syrup like consistency.