Heat 2 inches vegetable oil (about 3 1/2 cups) in a 10-inch cast-iron skillet until a deep-fry thermometer registers 375 degrees F. Soak the sliced fennel in a bowl of ice water; set aside.
Toss the scallops with 1/2 cup buttermilk and a pinch of salt and pepper in a bowl; set aside. Make the dressing: Puree the remaining 1/4 cup buttermilk and 2 tablespoons vegetable oil with the parsley, vinegar, 1/2 teaspoon salt and a few grinds of pepper in a blender until smooth; set aside. Mix the cornmeal, flour and 1/2 teaspoon salt in a shallow baking dish.
Drain the scallops from the buttermilk, dredge in the cornmeal mixture to coat and transfer to a plate. Fry the scallops in two batches until golden brown, 3 to 4 minutes. Remove with a slotted spoon and transfer to a rack set on a baking sheet; season with salt. Return the oil to 375 degrees F between batches.
Drain the fennel and pat dry with paper towels. Combine the fennel, lettuce, radishes and chives in a large bowl. Add the dressing and toss; season with salt and pepper. Divide the salad among bowls and top with the scallops and fennel fronds.