This fast and fresh seafood salad is a great meal for lazy beach days or busy weeknights. Choose fresh, not frozen, scallops for the best results. Can’t find Little Gem lettuce? Substitute mixed baby greens or chopped romaine lettuce.
Whisk together buttermilk, mayonnaise, white wine vinegar, tarragon, garlic, 1/4 teaspoon of the salt, and 1/8 teaspoon of the pepper in a small bowl. Slowly drizzle oil into mixture, whisking constantly, until combined.
Measure 2 tablespoons of buttermilk dressing into a medium bowl. (Reserve the remaining dressing for serving.) Add scallops to bowl, and toss to coat. Marinate at room temperature 10 minutes. Thread scallops onto skewers (about 5 per skewer). Discard marinade.
Preheat a gas grill to high (450°F to 500°F). Place mango planks on oiled grates. Grill, uncovered, until slightly charred, 1 to 2 minutes per side. Remove mango from grill; cut into 1/2-inch cubes. Set aside. Re-oil grates; place scallop skewers on grill. Grill, uncovered, until charred, about 1 minute per side.
Line a platter with lettuce. Top with mango, avocado, radish, and scallop skewers. Sprinkle salad with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Drizzle with reserved dressing, and serve immediately.