Take the rump roast out of the refrigerator approximately two hours prior to cooking, ensuring it acclimates closer to room temperature without exceeding the time limit.

Employ a slender, sharp knife to pierce three deep slits (roughly 2 to 3 inches) on one face of the meat. Into each incision, insert a garlic clove along with a pinch of sea salt and black pepper. Repeat the procedure on the opposite side, using the remaining garlic and seasonings. Generously rub any leftover salt and pepper over the entire surface of the meat.

Set the oven to preheat at 275°F.

In an oven-safe skillet, melt the salted butter over medium-high heat and sear the rump roast on all sides to form a golden-brown crust, which should take about 15 minutes. Transfer the seared meat to a plate.

Deglaze the skillet with beef broth and red wine, stirring to loosen and incorporate the browned bits.

Return the roast to the skillet, surrounding it with thin slices of onion and fresh rosemary sprigs.

Roast the meat in the oven until its internal temperature reaches 115°F. For a 4-pound roast, this will require approximately 12 to 15 minutes per pound, totaling around 48 to 60 minutes.

Once removed from the oven, place the roast on a cutting board, loosely cover it with foil, and allow it to rest for 20 minutes. Serve once the internal temperature registers 130°F for medium-rare or 125°F for rare.