Warm a skillet over medium-low heat, then melt a small amount of butter. Add the thinly sliced onions and cook gently, stirring occasionally, for about 20 minutes until they turn deeply caramelized and glossy.

While the onions are caramelizing, thinly slice your chosen apple. Set the slices aside to maintain their crunch and freshness.

Generously butter one side of each slice of sourdough bread. Place one slice, butter-side down, in your skillet.

Layer sharp cheddar and Gruyère cheese, followed by the caramelized onions, apple slices, and a sprinkle of salt, pepper, and optional thyme on the bread in the skillet. Top with a second slice of bread, butter-side up.

Cook the sandwich over medium-low heat until the bottom is golden brown, then carefully flip and continue cooking until the cheese is perfectly melted and the other side is equally golden.

Once done, transfer the sandwich from the skillet, let it rest for a moment, then slice it in half and serve while warm for maximum gooeyness.