Gourmet Seared Tuna Melt Sourdough Sandwiches

Ingredients

• 1 tablespoon minced red onion
• 1 tablespoon finely chopped celery
• 2 fresh tuna steaks (4 ounces each)
• 4 tablespoons mayonnaise, divided
• 4 slices sharp cheddar cheese
• 4 slices sourdough bread
• 1 tablespoon dill pickle relish
• 1/2 teaspoon freshly squeezed lemon juice
• 1 teaspoon fresh lemon zest
• salt and freshly ground black pepper to taste