In a small saucepan, prepare the au jus per package instructions. Add the roast beef and cook over low heat, just until warmed through, about 2-3 minutes. Do not boil.
Slice the baguette in half lengthwise. Spread the bottom half with the horseradish and the top half with the mustard.
Lay the cheese on both halves and place in a toaster oven to melt. Pile the roast beef on the bottom half, then top with the onion and the top bread half.
Slice in half on a bias and serve with the au jus on the side for dipping and the onion rings.
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