In 10-inch cast-iron skillet, melt butter over medium-low heat. Gently place onion slices close together in the butter. If last ring looks too large to fit in skillet, remove outside rings until small enough to fit. Sprinkle with salt, and cook 3 to 4 minutes without moving, until onions soften but don’t fall apart. Remove from heat. With large, flat spatula, gently turn onions, keeping rings together as much as possible.