Heat the oil in an 11- to 12-inch skillet (preferably cast iron) over high heat until shimmering hot. Arrange the squash in the skillet, cut side down and in a single layer. Season with 1/4 tsp. salt and a few grinds of pepper. Sprinkle the rosemary over the squash and sear the squash undisturbed until deep golden brown, 3 to 5 minutes. Using tongs, turn the squash onto the other cut side. Sprinkle with another 1/4 tsp. salt and cook until tender and nicely browned on the second side, about 2 minutes more. Transfer the squash and crisp rosemary to plates or a serving bowl.