Wash, dry, and slice tomatoes into ⅜-inch slices. Dredge them in cornmeal, and sprinkle both sides with salt and pepper. Heat oil in a large, heavy skillet over moderate heat. Add the tomato slices, without crowding. When the tomatoes start to sizzle, turn and brown on the other side. They should be just tender – not mushy. Remove slices as they are done and keep warm in a 175 degree F oven with the door slightly open. Serve warm, with a dollop of sour cream and a sprinkling of scallions.