Step 1

For the horseradish sauce: Combine the sour cream, horseradish, salt and pepper in a medium bowl. Cover and refrigerate to develop the flavors.

Step 2

For the au jus: Combine the onion soup, beef broth, sugar, Worcestershire sauce, salt and 3/4 cold water in a saucepan and bring to a boil over medium-high heat.

Step 3

Reduce heat and simmer 5 minutes. Taste and season with pepper and more salt if needed. Strain the mixture and discard the onions (if there were any in the soup.) Cover and keep warm.

Step 4

For the steak: Heat a grill or grill pan over high heat. Oil the grill pan if needed. Season the steak with salt and lots of black pepper. Grill the steak for 2 to 3 minutes per side for medium rare. Let the steak rest, then thinly slice.

Step 5

Spread the horseradish sauce on one side of each baguette. Dip the steak slices in the warm au jus and place on top of the bread with horseradish sauce, then place the other slices of bread on top. Transfer the remaining au jus to two dipping bowls and serve with the sandwiches for dipping.