Melt 1 tbsp butter over medium-low heat.
Add the sliced onions, brown sugar and a pinch of salt. Cook, stirring occasionally, until deeply golden and caramelized, about 45–60+ mins.
Mix all Garlic Butter ingredients together.
Add all Au Jus ingredients to a pot and simmer for 15–20 mins.
Warm the shaved roast beef in the hot Au Jus for 30–60 seconds.
Slice the loaf lengthwise. Spread with Garlic Butter, then top with beef, caramelized onions and provolone.
Bake at 400°F for 12 mins, then broil for 2 mins until the cheese is golden.
Slice into portions and serve with the warm Au Jus for dipping.
