Preheat the oven to 400°F. Peel and slice the onion in half lengthwise.

Place in a bowl and generously drizzle with olive oil. Sprinkle with kosher salt and pinch of sugar and toss with your fingers to combine.

Place the onions cut-side down in a large cast-iron or oven-safe skillet set over medium-high heat. Cook for 10 minutes, flips, then slide the skillet in the oven and bake for 30-40 minutes or until the onions are tender and caramelized.