Heat oil in a large cast iron pan, about a ½-inch high. Add the butter and mix in, it gives flavor and helps with browning. When hot, but not smoking, add tomato slices to skillet, and working in batches, fry until golden, about 3-4 minutes a side. Transfer to paper towel-lined plate to drain and sprinkle with salt while hot. Transfer fried slices to sheet pan with rack and keep warm in oven while continuing to fry remaining slices.