Preheat the oven to high broil and position the rack in the center. Slice the buns in half and place them on a sheet pan.
Spread softened butter on the insides of the rolls and sprinkle with garlic powder. Broil until lightly toasted (30–60 seconds). Remove the top buns and set them aside, keeping the bottom buns on the sheet pan.
In a large skillet over medium heat, add olive oil and 1 tablespoon butter. Once butter is melted, add the onion and garlic. Sauté for 5–7 minutes until golden, reducing the heat if necessary to prevent burning.
Sprinkle in the flour and cook, stirring constantly, for 1 minute. Gradually pour in beef consommé while whisking constantly. Bring to a boil over high heat, then reduce heat to low and simmer. Mix in the thyme and remove from heat.
Add the deli roast beef to the au jus sauce, stirring to coat the beef in the sauce. Using tongs, pull the beef from the liquid and portion equal amounts onto the toasted buns.
Cut the cheese slices in half and place them on top of the meat. Broil for 1–3 minutes, or until the cheese is melted. Remove from oven and press on the top bun.
Pour the remaining liquid into small bowls and serve alongside the hot sandwiches for dipping.
