Remove the roast from the fridge and place it on a wire rack and baking sheet or roasting pan with baking sheet. Season equally on all sides with salt, pepper, garlic and onion powder. Leave the roast out for at least 90 minutes before roasting.
Preheat the oven to 500°F. Make sure the fat cap is facing up. Insert an oven-safe digital thermometer into the center of the roast.
Pat the roast dry of any water that has wicked to the top of the roast. Place the roast in the oven and cook for 15 minutes then drop the heat to 300°F and continue cooking until the center of the roast reaches 110-115°F for rare, 125°F for medium rare, or 135°F for medium. The roast will take another 30-50 more minutes to cook (depending on doneness) after dropping the heat to 300°F.
Once the roast has finished cooking, move it to another wire rack to cool for 1 hour.
After 1 hour of cooling, wrap the roast with plastic wrap and place in the freezer for at least 30 minutes.
Place the roasting pan on a burner and turn the heat to medium-high. Add the sherry and a 1/2 cup of beef stock. With a wooden spoon scrape the pan to dislodge all of the brown bits. Turn off the heat and pour the pan's contents into the remaining beef stock and set aside.
Heat the butter in a saucepot to medium and add the onions. Saute the onions until they begin to turn brown on the edges (about 10 minutes).
Add the flour and cook for 2 minutes, stirring to incorporate. Add the beef stock, Worcestershire sauce, black pepper, garlic powder, and onion powder and bring to a boil while scraping the pan with a wooden spoon to dislodge any brown bits. Reduce by 1/3.
Once reduced, taste test and check the beefiness. If the jus needs more beef flavor, dissolve 1 teaspoon of reduced-sodium beef base into the pot and taste again. Once the flavor is to your liking season with black pepper and if needed salt to taste.
Slice the rolls and place onto a baking sheet. If the rolls are cold, they can be warmed up in a 350°F for a few minutes.
Use roughly 1/3 pound of beef per sandwich. Dip the beef into the hot broth to warm it through and place onto a roll. Lay 2 slices of cheese on top of each sandwich and serve with a side of jus for dipping.