Heat the oven to 400 degrees. With a sharp knife cut the top 1/2" off the flowering end of the garlic (not the root). Place the garlic in a square of tin foil. Drizzle the olive oil over the exposed cloves and seal the tin foil around the head of garlic. Bake for 25 minutes until soft. Set aside.
Meanwhile, fill a large pot halfway with cool water. Transfer the rutabaga to the pot. Add kosher salt and bring to a boil. Reduce heat and continue to simmer for 25 minutes. Add the parsnips and cook an additional 12-15 minutes until tender. Drain vegetables and transfer to a food processor. Add the vegetable stock and puree until smooth, scraping the sides occasionally (this may take several minutes). Squeeze the roasted garlic cloves to release the soft interior and add them to the puree. Add 6 tablespoons buttermilk and puree until both garlic and buttermilk are fully incorporated -- add more buttermilk by the tablespoon if the mash appears dry and pulse. Set aside. (Can be made in advance and reheated at the last minute).
FOR THE GREEN HERB SAUCE:
Add the parsley, cilantro, garlic, lemon zest, salt and pepper to the blender. Pulse several times to chop the herbs, scraping sides with a spatula. Add the lemon juice, mustard and olive oil and blend until emulsified. Pour into a small bowl or pitcher and set aside. (Can be made in advance. Serve at room temperature.)
FOR SEARED DIVER SCALLOPS:
Heat a dry skillet over medium high heat. Sprinkle scallops lightly with salt and pepper. When the pan is hot, add the olive oil -- it should slick across the surface. If it smokes immediately, remove it from the flame for 20-30 seconds to cool it. Carefully add the scallops 2-3 at a time and cook for 1-2 minutes to sear it. Using tongs, flip the scallops and cook for an additional 45 seconds to a minute. Transfer the scallops to a plate and continue to cook the remaining scallops in the same manner.
Place one or two scoops of the mash on the plate. Top with 3 seared scallops and drizzle the herb dressing over the scallops and vegetables. Serve.
Many markets sell diver scallops - but beware. Ask the purveyor if they are wetor dry scallops? Wet scallops have been soaked in phosphates to preserve them, but the scallops also soak up additional water, making it impossible to get a good sear. Dry scallops are never soaked and will produce a beautiful sear every time. Avoid wet scallops if you can.