Preheat oven to 375°F (190°C). Lightly butter the inside of each hoagie roll with the softened butter, then sprinkle each with a little garlic powder and a pinch of salt and pepper. Place rolls open-faced on a baking sheet.

Warm roast beef and toast rolls

If the roast beef is cold, warm it briefly so it's hot but not dry: place slices in a shallow oven-safe dish, cover tightly with foil, and warm in the oven for 6–8 minutes (alternatively microwave in a single layer covered with a damp paper towel for 45–60 seconds). While the beef is warming, place the baking sheet with buttered rolls in the oven and toast for 5–7 minutes until lightly golden and crisp at the edges.

Assemble sandwiches

Divide the warmed roast beef evenly among the rolls, piling meat loosely to keep it tender. If using cheese, place one slice over the meat and return sandwiches to the oven (or under a broiler) for 1–2 minutes to melt the cheese. Remove and keep warm. Keep the sandwiches covered loosely with foil while finishing the au jus.

Au Jus (Dipping Sauce)

Sauté aromatics

In a medium saucepan over medium heat, add 1 tablespoon neutral oil (or a splash of beef broth if avoiding oil). Add the sliced onion and smashed garlic and sauté until softened and lightly browned, about 6–8 minutes.

Deglaze and simmer

Pour in the red wine (if using) to deglaze the pan, scraping up any browned bits, and let it reduce by half, about 2 minutes. Add the beef broth, beef base (if using), Worcestershire sauce, soy sauce, thyme and bay leaf. Bring to a gentle simmer and let it cook uncovered for 10–12 minutes to concentrate flavors.

Finish and adjust

Taste the au jus and season with salt and pepper as needed. For a richer mouthfeel, whisk in 1 tablespoon cold unsalted butter until melted. If you prefer a slightly thicker jus, mix 1 teaspoon cornstarch with 1 tablespoon cold water to make a slurry, then whisk it into the simmering liquid and cook 1–2 minutes until slightly thickened. Remove and discard thyme stems and bay leaf. Keep the jus hot for serving.

Strain (optional)

For a smooth au jus, strain the sauce through a fine-mesh sieve into a heatproof bowl or small pitcher, pressing on the solids to extract all liquid. Return to the saucepan to stay warm if needed.