I heat a touch of olive oil in a small skillet and add my sliced red onion. Cooking it low and slow for about five minutes brings out a soft, jammy sweetness. This step is optional, but I never skip it because it adds so much flavor.
I lay out my sourdough slices and butter one side of each generously. On the unbuttered side, I spread a good layer of fig jam so every bite gets that sweet flavor. This little step makes a big difference in taste.
I place the brie slices directly onto the jam, then add my apple slices, caramelized onion, and a handful of arugula. Topping it with the second bread slice, butter side facing out, completes the sandwich before it hits the pan.
I heat my skillet over medium heat and place the sandwich butter side down. Cooking for about three minutes per side gets the bread golden and crisp while the brie turns soft and melty inside. I press gently with my spatula for even contact.
Once both sides are golden, I remove the sandwich and let it rest for a minute so the cheese sets slightly. Then I slice it in half on a diagonal for that classic sandwich-shop presentation and serve it warm right away.