1. For the seared watermelon tuna, into a medium skillet over high heat, add olive oil. When on the verge of smoking, carefully add watermelon, standing back to avoid being splattered. Cook watermelon slices, pressing firmly with a spatula to compress 2 to 3 minutes or until caramelized on one side. Flip and repeat on the other side.
2. For the classic vinaigrette, into a small bowl, add all ingredients whisk, slowly streaming in oil last to emulsify.
3. For the Niçoise salad, lay down a base of roughly torn or lightly chopped lettuce on a large serving platter and arrange green beans, tomatoes, olives, radishes, beans, and cucumber in rows. Top with seared watermelon tuna, drizzle with vinaigrette, and top with basil. Serve immediately.