Tuna Burgers with Wasabi Mayo and Quick Cucumber PIckle

Name: Tuna Burgers with Wasabi Mayo and Quick Cucumber PIckle
Ingredients: 1 English cucumber, peeled; 1/4 Cup cider vinegar; 1/4 Cup sugar; 3/4 Teaspoons salt; 10 Ounces ahi tuna steak, finely diced; 3 Tablespoons finely diced celery; 3 Tablespoons finely diced red bell pepper; 2 Tablespoons minced cilantro; 1 clove garlic, mincd; 1 large egg, beaten; 1 Cup panko breadcrumbs; 2 Teaspoons fresh lemon juice; Freshly ground black pepper, to taste; Pinch of cayenne; 2 hamburger buns, split; 1 Tablespoon unsalted butter, melted; 1/4 Cup mayonnaise; 2 Teaspoons wasabi powder, or to taste; 2 Tablespoons vegetable oil; 1/2 small ripe avocado, peeled and sliced; 6 large arugula leaves