In a large skillet over medium-high heat, add the lean ground beef and diced onions. Cook for about 8–10 minutes, stirring frequently, until the meat is browned and the onions are translucent. Drain any excess fat.

Reduce the heat to medium and introduce the unsalted butter, garlic powder, ranch seasoning, and au jus seasoning into the skillet. Stir these ingredients together, allowing the butter to melt and the spices to coat the meat evenly. Add the sliced pepperoncini peppers and some of the juice from the jar; heat for another minute.

Pour in the low sodium beef broth, stirring well to combine. Cook for an additional 3–5 minutes, stirring frequently.

While the meat filling simmers, heat a little olive oil over medium heat in another skillet. Cut the brioche buns in half and toast for about 2–3 minutes cut-side down.

Spoon the rich and tangy filling onto each toasted bun. Top with coleslaw if desired and serve immediately.