?1. In a small bowl or in the bowl of a min-food processor, combine buttermilk, mayonnaise, and yogurt. Add scallion, parsley lemon juice, mustard, and garlic; stir or pulse to combine. Season with salt and pepper; set aside.
2. Remove any wilted or torn outer leaves from lettuce. Using a chef’s knife, halve each head of lettuce lengthwise; rinse heads well under cold running water, taking extra care to flush water between leaves to remove any soil. Place lettuce, cut side down on a clean kitchen towel; pay as dry as possible. (If there’s any water remaining, the lettuce steams instead of charring on the grill.)
3. Heat a grill pan over high heat until very hot. Add halved lettuce, cut side down; grill until charred and lettuce wilts slightly, about 90 seconds.
4. Place two lettuce halves on each plate; spoon 1 ½ Tbsp dressing onto each plate. Scatter 1/8 of radishes over each salad, top with 1 Tbsp cheese, and season with black pepper, if desired. Serve immediately.