Heat a large cast-iron skillet over high heat. Rub 2 tsp of oil all over the halved squash. When the skillet is smoking hot, put in the squash cut side down. Let them sit, pressing lightly with a spatula, until seared and brown, 1-2 minutes. Flip and brown the curved sides too, another minute. Transfer the squash to a cutting board. Turn off the heat but leave the skillet on the stove.
When the squash is cool enough to handle, cut them into 1/4 inch thick slices. Heat the remaining 2 tbsp oil in the same skillet over medium heat. Add the onion and sage leaves and cook, stirring for 1 minute. Add the squash and a tsp salt. Reduce heat to medium low and cook, stirring occasionally until the squash is tender all the way through, 8-10 minutes. Serve hot.