Step 1Combine 6 ounces of the Gorgonzola and the vinegar in a medium bowl and mash them together with a fork until the cheese is smooth. Add the yogurt and buttermilk and use a whisk to stir the ingredients together. Add the lemon juice, thyme, garlic, salt and pepper, and stir to combine. Taste for seasoning and add more lemon juice, salt or pepper if desired. Break the remaining Gorgonzola into small pieces in the bowl and stir gently with a rubber spatula to incorporate the chunks into the dressing but not so much that they become too smooth. This makes a generous 3 cups of dressing, more than is needed for the recipe; the dressing will keep, up to 3 days, covered and refrigerated.
Step 1Place the onion slices in a small bowl of ice water and set them aside while you prepare the rest of the ingredients. Drain the onions and pat them dry with paper towels before adding them to the salad.
Step 2Remove the rough outer leaves from the lettuce and cut off and discard the rough caps at the bottom of the cores, leaving the heads intact. Starting at the core, cut each head of lettuce in half lengthwise.
Step 3If you are making 4 individual salads, spoon one-fourth cup of the dressing on each of 4 plates, otherwise spoon 1 cup of the dressing in the center of a long platter and use the back of a spoon to spread the dressing along the length of the platter. One by one, spoon 2 tablespoons of the dressing over each half head, using the back of the spoon to spread it along the cut sides of the lettuce halves, leaving the edges of the heads visible. As you dress the halves, place them faceup on the dressed plates, as the dressing on the plates will help keep the lettuce in place. For individual salads, place one lettuce head half, cut side up, on each plate, lean the second half against the first and the third against the second so they are stacked almost like a tepee. To build the salad on a platter, place the lettuce head halves along the length of the platter backgammon style, with one facing one direction and the one next to it facing the other direction.
Step 4Tear the dates into thin strips and scatter the strips over the salad. Scatter the onion slices over the salad, sprinkle with the thyme, grind pepper over the salad and serve.