Onion Relish

• 2 pounds large sweet onions, chopped
• 2 celery stalks, sliced
• 1 medium red bell pepper, chopped
• 1 tablespoon melted butter
• 3/4 cup apple cider vinegar
• 1/4 cup water
• 2 tablespoons brown sugar
• 1 teaspoon celery seed
• 1/4 teaspoon salt