Even lettuce can be grilled. Romaine stands up beautifully to heat and develops a nice sweetness. The salad, served with pickled radishes and shallots and buttermilk dressing, is best when still hot.
Place radishes and shallot in a small bowl. Bring 2/3 cups of water, 1/2 cup vinegar, the sugar, and 2 teaspoons salt to a boil in a small saucepan. Pour over radishes and shallot. Refrigerate.
Meanwhile, whisk together buttermilk, sour cream, and remaining 2 tablespoons vinegar. Season with salt and pepper. Refrigerate while grilling romaine.
Heat grill to medium-high. Brush romaine halves with oil, and season with salt and pepper. Grill, turning, until charred but still crisp, about 4 minutes per side. Cut each in half lengthwise. Drizzle dressing over romaine. Drain radish-shallot mixture. Scatter over romaine. Serve immediately.