Shuck oysters and set aside, reserving 12 shells (large “cup” sides). Scrub shells well. Place oysters and buttermilk in a medium bowl. Cover and let sit 20 minutes (or up to an hour). In a deep cast-iron pan (3- or 4-inch rim), heat 2 inches of oil to 375 degrees. In a separate bowl, combine cornmeal, flour, and a few pinches of salt and pepper. Remove oysters from buttermilk with a slotted spoon and place in flour mixture. Toss lightly, coating well, and shake off excess gently. Place in oil and fry until golden brown (about 2 minutes). Remove and place on paper towels. Season with salt to taste. Spoon 2 to 3 teaspoons of tartar sauce into each oyster shell. Place one oyster on top. Garnish with chopped scallions if you’d like and serve immediately.
In a medium bowl, whisk first seven ingredients to blend. Season to taste with salt and pepper. You may prepare sauce in advance and refrigerate it, covered, up to 2 days. If it releases some liquid, drain carefully before serving. Makes about 2 cups