Seared Tuna and Smoked Trout Sandwich

Name: Seared Tuna and Smoked Trout Sandwich
Ingredients: 3.5 ounces smoked trout; 2 teaspoons wasabi paste (or to taste); 1/2 cup mayonaisse; 1 tablespoon sweet pickle relish; 2 teaspoons Old Bay seasoning; coarse salt and black pepper (to taste); 1-2 fresh, raw tuna steaks (about 12 ounces); 1 tablespoon freshly squeezed lemon juice; 1 stalk celery, cut into 1/4-inch (or smaller) dice; 1 tablespoon minced chives; 8 slices rustic bread, lightly toasted; 4 large lettuce leaves (optional); 8 slices fresh tomato (optional)