Start by placing all of the marinade ingredients in a zip seal bag along with the beef and let marinate in the fridge for 3-4 hours.
Preheat your oven to 325 and place your beef with marinade in a roasting dish. Add onions to the pan and let cook for 1 hour and 15 minutes for medium rare depending on your oven and the roast.
Remove roast and let rest covered loosely with foil.
Add the beef stock to the onions and cook for 5 minutes. Mash the onions to release more flavor. Then strain and set aside.
In a small bowl combine the mayo and horseradish.
Slice the baguette in half and generously spread the horseradish mayo on each side. Slice the beef into thin slices and place on each baguette and topping with thin slices of the Swiss cheese. Broil the cheese side of the sandwiches until beautifully melted. Serve with the au jus on the side.