1. Preheat a large skillet over medium heat. Add the sliced mushrooms and sauté until they are soft and lightly browned, about 5-7 minutes. Remove from heat and set aside.

2.  In the same skillet, add a little bit of olive oil and sauté the sliced jalapeno for about 2-3 minutes, or until they are slightly softened. Remove from heat and set aside.

3. Preheat the broiler. Slice the hoagie roll in half and spread with garlic butter. Place the two halves on a baking sheet. Broil for 1-2 minutes, or until lightly toasted.

4. Preheat flat grill to 350°F. Place thawed portion of beef on grill. After the portion has cooked through about half its thickness, approximately 1-2 minutes, flip the portion once. Continue to cook the meat another 1-3 minutes. Meat should have firm, cooked appearance

6. Spread the horseradish mayo on the bottom half of the hoagie roll. Layer the beef on top of the mayo. Top the beef with the sautéed mushrooms and sliced jalapeno. Place the Swiss cheese slices on top and place the hoagie roll under the broiler for 1-2 minutes, or until the cheese is melted and bubbly.

7. While the sandwich is under the broiler, heat up the beef au jus in a small saucepan over low heat.

8. To serve, top with crispy fried onion and  cut the sandwich in half and serve with a small cup of the au jus gravy on the side for dipping. Enjoy your delicious Philly-style sandwich!