• 4 cups mixed bell peppers, thinly sliced.
• 2 large onions, thinly sliced.
• 1 cup apple cider vinegar.
• 1 cup granulated sugar.
• 1 tablespoon mustard seeds.
• 1 teaspoon celery seeds.
• 1 teaspoon turmeric powder.
• 1/2 teaspoon ground black pepper.
• 1/2 teaspoon red pepper flakes (optional).