Leftover Prime Rib French Dip Sandwiches

Instructions

1. Start the au jus. In a medium saucepan over medium heat, whisk together all au jus ingredients except the butter: beef broth, drippings (if using), Worcestershire, soy sauce, Dijon (if using), garlic, and black pepper. Bring to a gentle simmer, then lower heat to keep it hot. Taste and salt as needed (broth and drippings vary a lot). Whisk in the cold butter at the end, if using, for a glossy finish.
2. Quick-brown the onions. Heat a large skillet over medium-high heat. Add olive oil and butter. Add sliced onions, salt, and pepper. Cook 8 to 10 minutes, stirring often, until the onions are soft and translucent with browned edges. If they start to scorch, reduce heat to medium. If the pan gets dry, add a splash of water and scrape up the browned bits.
3. Toast the rolls. Push onions to one side of the skillet or move them to a plate. Place rolls cut-side down in the skillet for 1 to 2 minutes until golden. If you want extra crunch, add a tiny smear of butter first.
4. Warm the prime rib gently. Turn the au jus down so it is hot but not boiling. Use tongs to dip a few slices of prime rib into the au jus for 20 to 40 seconds, just until warmed through.
5. Assemble and melt (if using cheese). Pile warm beef onto each toasted roll and top with onions. Add cheese if you want. For extra-melty cheese, set the sandwiches (open-faced) on a sheet pan and broil for 30 to 60 seconds, just until melted. Or cover the skillet with a lid for 30 to 60 seconds with the sandwiches inside, keeping the heat low.
6. Serve with dip. Ladle au jus into small bowls (about 1/2 to 3/4 cup per person) and serve immediately. Optional but recommended: a swipe of horseradish mayo on the roll before the beef goes on.