How to Make It
Preheat the oven to 350°. Place the head of garlic, cut side up, on a double sheet of foil and drizzle with olive oil. Wrap the foil around the garlic and bake until the cloves are very soft, about 1 hour. Let cool.
Light a grill. Squeeze the garlic cloves into a blender. Add the buttermilk and lemon juice and blend to a puree. With the machine on, slowly pour in the 1 cup of olive oil until blended. Season the dressing with salt and pepper.
Drizzle the cut side of the romaine hearts with olive oil and sprinkle with salt and pepper. Grill the romaine over a hot fire, cut side down, until charred in spots, about 20 seconds. Turn the romaine over and grill for 20 seconds longer. Transfer the heads to a platter, cut side up. Drizzle the dressing over the romaine, sprinkle with the Manchego and serve right away.