|4 cup • Beef stock|
|1/2 • White onion • roughly chopped|
|1 • Carrot • peeled, roughly chopped|
|2 • Celery ribs • roughly chopped|
|4 • 8-inch lengths of French bread • split lengthwise|
|8 slice • Provolone cheese|
|8 tbsp • Homemade Mustard|
|24 oz • Roast beef • thinly sliced|
|1 cup • Caramelized onions|
Position an oven rack in the center of the oven and preheat to 350 degrees F.
Place the broth, onion, carrot, and celery in a small pot and bring to a simmer. Turn the heat down to low and cover with a lid to keep warm.
Place a piece of parchment paper on a baking sheet and arrange the slices of bread, cut sides up, on the baking sheet. Using your fingers, gently remove some of the soft, inner part of the bread (if it is too thick or dense), leaving about a ½-inch thick shell. Spread 2 tablespoons of Homemade Mustard per sandwich along the top and bottom. Place two slices of provolone cheese along the bottom side of the sandwich and 6 ounces of roast beef. Drizzle a scant tablespoon of the broth over the beef. Transfer to the oven and cook until meat is just warmed through, about 6 minutes.
Remove from the oven and spread ¼ cup of the caramelized onions on top of the meat and place the sandwiches on serving plates. Carefully dip sandwiches on both sides into beef broth before serving or pour the beef broth into four serving ramekins and serve with the sandwiches for dipping.