In a 10- or 12-inch nonstick skillet, heat the butter and olive oil over medium heat until hot and rippling.

Add the zucchini and yellow squash in as even a layer as possible (it should sizzle as it hits the skillet) and sprinkle with salt and pepper. Let it sit without stirring or moving for 2-3 minutes so it can get nice and golden. Give it a good stir, add a touch more salt and pepper to taste if needed, and let it continue to cook, stirring only every now and then, until the squash is browning here and there and tender to your liking, about 5-7 more minutes.

Season with salt and pepper to taste and top with freshly grated Parmesan, if desired. Serve immediately.