Preparation

Make the dijonnaise:

• Heat the oven to 350°F. Put the garlic in the center of a square of foil and arrange the rosemary sprigs on top. Drizzle the oil over the rosemary and garlic, and sprinkle with a pinch of salt. Gather the foil around the garlic into a pouch and put the pouch in a pie plate or other small baking dish (in case the oil leaks). Roast until the garlic is light golden and completely soft, about 30 minutes. Open the foil and let cool briefly. Discard the rosemary sprigs. Transfer the garlic to a small bowl and reserve the oil in the foil. Mash the garlic with the back of a spoon, and then stir in the mayonnaise, mustard, 1/2 tsp. of the garlic-rosemary oil, and a small pinch of salt.

Assemble the sandwiches:

• Position a rack 3 to 4 inches from the broiler element and heat the broiler on high. Slice the rolls in half, set cut sides up on a baking sheet, and brush with the remaining rosemary-garlic oil. Toast under the broiler until golden brown, about 1 minute. Spread some Dijonnaise on the top and bottom of each roll. Arrange the beef over the bottom halves along with the sautéed onions and mushrooms and replace the tops.