Tuna Melt Sandwich

Name: Tuna Melt Sandwich
Prep time: 15 min
Total time: 15 min
Cuisine: American
Ingredients: 2 (5-ounce) cans chunk white tuna, drained; 1 celery stalk, chopped; 1/3 c. chopped fresh flat-leaf parsley; 1/3 c. chopped fresh dill; 2 scallions, thinly sliced; 1 tbsp. fresh lemon juice; 1/2 c. mayonnaise; 1/4 c. sliced cornichons; 2 tbsp. pickle relish; 1 tbsp. Dijon mustard; Kosher salt and freshly ground black pepper; 8 slices sourdough bread; 1/4 c. mayonnaise; 4 oz. extra-sharp Cheddar, grated (about 1 cup)