Preheat your oven to 400°F. Grease a square 8-inch Pyrex baking dish with butter.
Peel the onion and slice it into ¼-inch-thick slices. You should get five slices.
Arrange the slices in a single layer in the prepared baking dish. Melt the butter and pour it over the onions. Sprinkle them 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 golde brown, about 20 more minutes.
Serve immediately.
Video
Notes
You can 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. The edges should be deep brown, not black.
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, using ¼ teaspoon of each. You can also sprinkle them with grated parmesan cheese after flipping them.
You can keep the leftovers in the fridge, in a sealed container, for 3-4 days. Reheat them, covered, in the microwave at 50% power. Alternatively, chop the leftovers and mix them into an omelet or a scramble.