How to Make It
Make the dressing: Preheat broiler to high. Line a rimmed baking sheet with aluminum foil, and add sliced ramp or scallion bulbs and garlic, if using. Toss with oil and 1/4 teaspoon salt, and spread in an even layer. Broil until charred around the edges, 8 to 10 minutes. Let cool.
Whisk together mayonnaise, buttermilk, rice vinegar, lemon zest, lemon juice, 1 tablespoon dill, 1/2 teaspoon pepper, and remaining 1/2 teaspoon salt in a medium bowl. Finely chop reserved ramp or scallion greens to equal 1/2 cup. Fold chopped greens and charred ramps into mayonnaise mixture. Cover and chill until ready to serve.
Make the salad: Arrange lettuce leaves in a very large wooden salad bowl. Top with carrots, turnips, celery, radishes, tomatoes, avocados, and 2 tablespoons dill. Drizzle with dressing, and sprinkle with freshly ground black pepper.