Cut the French bread into sandwich-length pieces and slice the onion.
Heat the griddle to medium and add a drizzle of olive oil. Place the sliced onions onto the griddle and cook until lightly browned, stirring occasionally. Reduce the heat under the onions to low once they reach your desired color.
On a separate burner or camping stove, warm the au jus over low heat and keep it at a gentle simmer.
Place the French bread cut-side down on the griddle and press lightly to toast. Flip the bread, add a little more olive oil to the griddle, and toast the second side until golden, then turn off the heat under the bread.
Place an omelet ring on the griddle (optional) and add half of the roast beef inside the ring to warm. Add a layer of sharp cheddar cheese over the warmed beef. In the other side of the omelet ring, add the remaining roast beef followed by another layer of cheddar.
Cover the meat and cheese with a dome and allow the cheese to melt completely.
Spread prepared horseradish onto the toasted bread.
Use two spatulas to lift the melted roast beef stack from the griddle and onto the bread.
Add a portion of the browned onions over the first layer of roast beef.
Place the second layer of roast beef on top, followed by more browned onions.
Close the sandwich with the top piece of bread.
Pour the hot au jus into a bowl for dipping.
Serve the sandwich warm and enjoy with plenty of au jus.
