In a skillet over medium heat, heat oil and butter until shimmering. Add in the sliced onions and saute until translucent; 3 minutes. Reduce heat slightly, season onions with salt and continue to cook until onions are a deep golden brown colour; 25 to 30 minutes.
Preheat toaster oven to 350°F (177°C). Line toaster tray with foil or parchment paper.
Place about two inches of water into a medium pot. Bring to a simmer.
Place the Levitts Seasoned Roast Beef Thinly Sliced meat into a steam basket or metal sieve and place over the simmering pot of water, ensuring the steam basket or sieve is not immersed in the water. Cover with the lid or foil. Steam for 5 minutes. Remove from heat, and keep covered.
Spread mayonnaise on both sides of the roll. Evenly pile the warm slices of the seasoned roast beef in the roll. Top with slices of Provolone cheese and about a quarter to half the amount of the caramelized onions. Place sandwich onto the prepared tray and place into the preheated toaster oven until cheese begins to melt and roll is warm.
With a serrated bread knife or sharp chef’s knife, cut roll in half. Serve with au jus, if desired.