In a medium size sauce pot, heat the beef broth over medium heat. Cook the whole baguette in the oven for about 5 minutes, just to warm it up and start to crisp the outside. Remove the baguette from the oven, slice in half vertically, then slice each half horizontally. Place the slices cut side up on a parchment lined baking sheet, and evenly divide the shredded gruyere cheese on the sliced baguette. Return to the oven for another 5 minutes until the cheese is melted.
While the cheese is melting, add the roast beef to the beef broth. When the cheese is melted, assemble the sandwich by spreading the grilled onions on one half, and laying the beef broth soaked roast beef on the other half. Top the roast beef with arugula. Cut the sandwiches in half again so you now have 4 servings. Pour the remaining beef broth into bowls and serve with sandwich as an “au jus”.