Thai Tuna Burgers with Ginger-Lemon Mayonnaise

Thai Tuna Burgers:

• Kirby cucumbers, thinly sliced
• ¼ medium red onion, thinly sliced
• 3 tablespoons rice vinegar
• 1 tablespoon plus 1 teaspoon sugar
• Kosher salt and freshly ground pepper
• 2 teaspoons fresh ginger, finely grated
• 1 garlic clove, crushed
• 1 serrano chili, seeded and minced
• 2 tablespoons Asian fish sauce
• 2 tablespoons cilantro, finely chopped
• 1 tablespoon basil, finely chopped
• 1 ½ pounds sushi-quality tuna (available from grocery)
• 1 ½ tablespoons vegetable oil
• 1 ½ teaspoons Asian sesame oil
• 4 hamburger buns
• 2 tablespoons dry-roasted peanuts, finely chopped

Ginger-Lemon Mayonnaise:

• 1 cup mayonnaise
• 2 tablespoons soy sauce
• 2 tablespoons cilantro, finely chopped
• 2 tablespoons scallion greens, finely chopped
• 1 tablespoon toasted sesame seeds
• 2 teaspoons fresh ginger, finely grated
•  zest of 1 lemon, finely grated
• 2 teaspoons fresh lemon juice
• freshly ground pepper