To get started, gather your ingredients. Slice the onion thinly, about a quarter-inch thick. This thickness ensures they cook evenly without burning.

Heat a large skillet over medium heat, and then add in the butter. Let it melt completely, covering the base of the skillet with its rich, golden goodness.

Once the butter is hot, toss the sliced onions into the skillet. Use a spatula to stir the onions so they coat themselves in the melted butter. This is key to achieving that delightful caramelization.

Sprinkle the sea salt and brown sugar over the onions. The sugar not only adds sweetness but helps them caramelize faster. Stir gently to distribute the seasoning evenly.

Now, let the magic happen. Cook the onions for about 15-20 minutes, stirring occasionally. If you notice them browning too quickly, lower the heat. You want them to turn a deep golden brown, not burn.

If the onions begin to stick to the pan, add a splash of water. This helps in loosening them while continuing to cook.

After about 20 minutes, you should have beautifully caramelized onions. If using, toss in a sprig of fresh thyme during the last minute of cooking to incorporate herby notes.

Once your grilled onions are perfectly done, remove them from the heat. They’re ready to top off your burgers, or you can serve them as a side. Their flavor will shine through every dish you prepare.