Steak Sandwich

Bahan-bahan (4 porsi)

• 1 (16-ounce) 1-inch-thick New York strip boneless beef top loin steak (or hangar steak* )
• Kosher salt
• Ground black pepper
• 2 ½ tablespoons extra-virgin olive oil (divided)
• 2 large red onions (thinly sliced)
• 1 tablespoon Dijon mustard
• 2 tablespoons unsalted butter (melted and divided)
• 2 large (or 4 medium) focaccia or ciabatta buns
• 1 cup baby arugula
• 6 ounces Brie cheese (cut into thin slices)
• ½ cup fig jam