Step 1: Season and Sear the Roast

Start by patting the 3.5lb rump roast dry with paper towels to ensure a good sear. Generously season all sides with salt, pepper, and 1 Tbsp of dry Italian seasoning. The salt enhances the natural flavors while pepper adds a subtle kick, and the Italian seasoning brings herbaceous notes of oregano, basil, and thyme that infuse the meat during cooking.

Heat 2 Tbsp of vegetable oil in a large skillet over high heat until it shimmers and almost smokes. This high temperature creates a beautiful crust through the Maillard reaction, locking in juices and building deep flavor. Place the seasoned roast fat-side down first, then sear for 2 to 3 minutes per side, using tongs to turn it carefully. Aim for a rich brown crust on all sides without cooking the interior. Remove the roast to a clean plate and set aside; the pan will have flavorful browned bits called fond that you will use next.

Step 2: Sauté the Onions

With the skillet still hot, add the sliced yellow onions if needed with a touch more oil. Increase heat to high to develop color quickly. Stir occasionally for 6 to 7 minutes until the onions turn golden brown. This caramelization process converts sugars in the onions, adding sweetness and complexity that elevates the au jus beyond basic broth.

Avoid overcrowding the pan, as this steams rather than browns the onions. Thicker slices, about 1/4-inch, prevent them from disintegrating during the long slow cook. Once browned, transfer the onions to the bottom of your slow cooker insert. Their aroma alone hints at the deliciousness to come.

Step 3: Build the Au Jus Mixture

In a bowl or directly in the skillet, combine 2 cups of beef stock (prepared with beef base and hot water if using concentrate), 1 cup dark beer like a lager, 2 Tbsp Worcestershire sauce, 2 Tbsp soy sauce, 2 Tbsp dried onion flakes, chopped garlic from 8 cloves, and a generous grind of black pepper. Stir to blend; no salt yet, as the soy and Worcestershire provide plenty.

The dark beer adds malty depth and tenderizes the meat with its mild acidity, while dried onion flakes mimic onion soup mix for umami without packets. Pour this mixture over the onions in the slow cooker, distributing evenly. The garlic will soften and sweeten over time, melding into the savory broth.​

Step 4: Slow Cook the Roast

Nestle the seared rump roast on top of the onions in the slow cooker. Cover and set to low for 6 to 8 hours, or high for 4 to 6 hours if rushed. Low and slow breaks down tough connective tissues in the rump roast into gelatin, yielding fork-tender meat that shreds or slices effortlessly.

Your home will fill with intoxicating aromas of beef, beer, and herbs after just an hour. Resist peeking too often to maintain steady heat. After cooking, the roast should reach at least 205°F internally for optimal tenderness.

Step 5: Rest and Prepare the Beef

Remove the roast to a cutting board and tent loosely with foil for 20 minutes. This resting period redistributes juices. Skim excess fat from the au jus surface with a ladle; reserve the defatted broth as your dipping sauce.

Slice the roast against the grain into thin pieces for even texture, or shred with forks for a pulled style. Return slices or shreds to the warm au jus to soak for 5 to 10 minutes, absorbing flavors without sogginess.

Step 6: Make the Horseradish Sauce

In a bowl, whisk together 1/2 cup mayo, 1/4 cup sour cream, 1 Tbsp prepared horseradish (adjust for heat), 2 Tbsp chopped chives, 1 Tbsp whole grain mustard, 2 tsp Dijon mustard, a dash of Worcestershire, and black pepper. Taste and add more horseradish for zing.

This sauce cuts through the richness with creamy sharpness; chives add freshness, mustards bring tang. Chill until serving to let flavors marry.​

Step 7: Toast the Rolls

Preheat your broiler. Brush insides of 6 crusty 8-inch rolls with melted butter for golden crispness. Place open-faced under the broiler for 1 to 2 minutes until toasted. Watch closely to avoid burning.​

Butter prevents sogginess and enhances flavor. Crusty rolls hold up to juices better than soft bread.​

Step 8: Assemble the Sandwiches

Drain beef slightly and pile onto toasted roll bottoms with some onions. Top with sliced provolone. Broil 30 to 60 seconds until cheese bubbles and melts.

Cap with roll tops, pressing gently. Slice in half if desired. Serve au jus in bowls for dipping, horseradish sauce on side.