Wash, dry and slice the tomatoes into 3/8-inch thick. Transfer the tomatoes to a paper towel to drain.
Combine the cornmeal, panko bread crumbs, 1 teaspoon salt, pepper, and cayenne in a shallow bowl.
In a second shallow bowl, combine the buttermilk and half of the yogurt.
Meanwhile, combine the sour cream, remaining yogurt, horseradish, lemon zest, lemon juice and 1/2 teaspoon of the salt to make the horseradish cream. Cover and refrigerate until serving.
Heat the oil in a cast iron pan to 365 degrees F.
Dip the sliced tomatoes in the buttermilk mixture and shake off any excess. Press them into the panko mixture and repeat until all the tomato slices have been coated.
Carefully lower the battered slices, in batches, into the hot oil. Cook for 2 to 3 minutes on both sides, then move them to a wire rack to drain. Sprinkle them with remaining salt while they are still hot. Arrange them on a serving platter and serve immediately with the horseradish cream.