Heat the oil in a pot on medium heat. Saute the onions until softened and starting to brown. Then stir in the broth, beef, and rosemary. Bring to a low simmer until the flavor concentrates, about 15 to 20 minutes. Then add in the butter. Taste and season with salt and pepper to taste.
Preheat the oven broiler on low. Slice open the rolls and place them on a baking sheet. Toast the bread under the broiler until golden brown (keep a close eye on the bread so that it doesn't burn).
Using tongs or a slotted spoon, place the beef on the roll.
Serve the sandwich with a side of au jus for dipping.
Enjoy the french dip with pita chips and hummus, pickle, and apple cider.