French Dip Sandwich with Caramelized Onions

Ingredients:

• 1 flank steak weighing 900 g.
• 2 tbsp tomato paste
• 2 tablespoons balsamic vinegar
• 2 cups lightly salted beef broth
• 1/4 cup Worcestershire sauce
• 4 sprigs fresh thyme + 0.5 tsp thyme leaves, finely chopped
• 2 cloves garlic, crushed
• 1 tbsp unsalted butter
• 1 small red onion, thinly sliced ​​(about 1 cup)
• 4 soft hoagie buns, lightly toasted

Preparation:

1. Pierce the steak several times with a fork and place it in a zip-lock plastic bag. In a small bowl, combine the tomato paste, balsamic vinegar, 1/2 cup beef broth, and 2 tablespoons Worcestershire sauce until smooth. Pour the marinade over the beef and add the thyme sprigs and minced garlic. Seal the bag and tamp the meat until it's coated in the marinade. Marinate in the refrigerator for 4-6 hours.
2. Preheat the oven to broil. Line the bottom of a roasting pan with a rack or a rimmed baking sheet with heavy-duty aluminum foil. If using a baking sheet, place a rack on top.
3. Remove the meat from the bag and discard the marinade. Pat the meat dry and sprinkle with 1 teaspoon salt and 1/2 teaspoon black pepper. Pour the remaining 1 1/2 cups beef broth into the bottom of the prepared baking sheet and place the meat on the rack. Grill, turning once, until an instant-read thermometer inserted into the center of the steak registers 125°F (52°C), about 6 minutes per side for medium-rare. Let the meat rest on the rack for 5 minutes, then transfer to a cutting board and let rest for another 5 minutes. Retain the juices from the pan.
4. Meanwhile, melt the butter in a medium skillet over medium heat. Add the onion, finely chopped thyme, and 1/4 teaspoon salt and cook, stirring, until the onion is very soft and lightly browned, about 10 minutes. Transfer to a bowl and set aside.
5. Pour the pan juices and the remaining 2 tablespoons Worcestershire sauce into a small saucepan and heat over medium heat.
6. Slice the meat thinly across the grain and lightly salt the slices. Divide the sliced ​​meat and fried onions among the hoagie buns. Serve with the dipping sauce.