The Dip Recipe

Prep the steak

If your sirloin is not already thinly sliced, place it in the freezer for 20–30 minutes to firm up, then slice across the grain as thinly as possible (about 1/8" to 1/4"). Pat the slices dry with paper towels.

In a large bowl, combine olive oil (2 tbsp), Worcestershire (1 tbsp), salt (1.5 tsp), black pepper (1 tsp) and garlic powder (1 tsp). Add the sliced steak and toss to coat. Let marinate at room temperature for 10–15 minutes while you prepare other components (do not marinate more than 30 minutes for thin slices).

Heat a large heavy skillet or cast-iron pan over high heat until very hot. Add 1 tbsp unsalted butter and swirl. Working in a single layer and in batches to avoid overcrowding, sear the marinated steak slices for 30–60 seconds per side until just browned (the thin slices will cook quickly). Remove each batch to a plate and tent loosely with foil. Reserve any fond (browned bits) in the pan — you'll use them for the au jus.

Cook the mushrooms

In the same skillet (wipe out if there is burnt residue but keep fond if usable), add 2 tbsp unsalted butter and 1 tbsp olive oil over medium-high heat. When butter foams, add the chopped shallot (1 each) and sauté for 1 minute until translucent.

Add the sliced mushrooms (8 oz) in a single layer if possible and let them sear without stirring for 2–3 minutes, then stir and continue to cook until golden brown, about 5–6 minutes total. Add minced garlic (1 clove), fresh thyme leaves (1 tsp), salt (1/2 tsp) and black pepper (1/4 tsp) and cook for another 30–45 seconds until fragrant. Remove mushrooms to a bowl and keep warm.

In a small bowl, whisk together sour cream (1/2 cup), prepared horseradish (2 tbsp), mayonnaise (2 tbsp), Dijon mustard (1 tsp), lemon juice (1 tsp), salt (1/4 tsp) and black pepper (1/8 tsp) until smooth. Taste and adjust horseradish or salt as desired. Refrigerate until assembly.

Build the au jus

In a saucepan over medium heat, melt 1 tbsp unsalted butter. Add the thinly sliced onion (1/2 each) and smashed garlic clove (1 clove); sauté until softened and fragrant, about 3–4 minutes.

If using, pour in the red wine (1/4 cup) to deglaze the pan, scraping up browned bits from the bottom; simmer 1–2 minutes to reduce slightly. Add low-sodium beef broth (2 cups), Worcestershire (1 tsp), and beef base (1 tbsp) if using. Add the thyme sprig. Bring to a simmer and let cook gently for 8–10 minutes to concentrate flavor.

Remove the thyme sprig and discard. If you prefer a slightly thicker au jus, make a slurry by whisking cornstarch (1 tsp) into cold water (2 tsp) until smooth, then whisk it into the simmering broth and cook 1–2 minutes until slightly thickened. Season with salt (1/4 tsp) and black pepper (1/8 tsp). Keep the au jus warm. Optional: strain the jus through a fine-mesh sieve for a clear sauce.

If you reserved pan fond from searing the steak, you can add that to the au jus while deglazing for extra beef flavor.

Prepare rolls and cheese

Preheat the broiler or oven to high (about 500°F/260°C). Split the rolls (4 each) and spread the cut sides lightly with the reserved 2 tbsp unsalted butter. Place cut-side up on a sheet pan and toast under the broiler 1–2 minutes until golden — watch carefully to avoid burning.

Arrange toasted roll bottoms on the sheet pan. Divide the warm steak evenly among the 4 rolls, piling it on the bottoms. Top the steak with the sautéed mushrooms, then place 2 slices of Swiss cheese on each sandwich.

Place the assembled sandwiches under the broiler just until the cheese melts, about 1–2 minutes. Alternatively, use a very hot oven or a salamander to melt the cheese.

Spread a generous spoonful of horseradish cream (about 1–2 tbsp) on the cut side of each toasted roll top. Place the tops on the melted-cheese sandwiches.

Ladle the warm au jus into individual small bowls or a communal dipping bowl. Sprinkle chopped parsley (1 tbsp) over sandwiches if using.

Serve each sandwich immediately with a side of au jus for dipping. To serve warm steak in the sandwich, you can briefly toss the seared steak back into a small portion of the hot au jus before piling on the rolls to keep it juicy.