Grease a square 8-inch Pyrex baking dish with butter.
Peel the onion and slice it into ¼-inch-thick slices, as shown in the video below. You will probably end up with five slices.
Arrange the slices in a single layer in the prepared baking dish. Melt the butter and pour it all over the onions. Sprinkle with Kosher salt.
Roast the onions until the bottoms are golden, about 20 minutes.
Turn the onion slices to the other side and bake them until golden-brown, about 20 more minutes.
You can easily double this recipe and use a 9 X 13 -inch Pyrex pan.Keep a close eye on the onions during the last 10 minutes of baking to ensure they don't burn. Your oven might run hotter than mine.Since the onions are so flavorful, I don't add any other seasonings. But if you'd like, you can season the onions with garlic powder and oregano. You can also sprinkle them with grated parmesan cheese after the first 20 minutes of cooking.