Gourmet Tuna Melts: A Grown-Up Twist on Canned Tuna Recipes

Ingredients

• 1/2 small red onion, thinly sliced
• 1/4 cup apple cider vinegar
• 1 tablespoon granulated sugar
• 1/4 teaspoon salt
• 2 (5-ounce) cans tuna in water, drained well
• 1/4 cup mayonnaise
• 1 tablespoon Dijon mustard
• 1 tablespoon chopped fresh parsley
• 1 tablespoon chopped celery
• 1/4 teaspoon black pepper
• Pinch of red pepper flakes (optional)
• 4 slices sourdough bread
• 4 ounces sharp cheddar cheese, shredded
• 2 tablespoons butter, softened