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.