Cover the casserole dish tightly with foil. This helps the beef braise gently in the soup and turn very tender, just the way my mother-in-law showed me.
Bake, covered, for 1 hour. Carefully remove the foil, then continue baking for another 15–20 minutes, until the beef is very tender and the onion gravy is bubbling and slightly thickened around the rolls.
Let the beef rolls rest for about 5 minutes before serving. Spoon some of the rich brown onion gravy from the dish over the top of each serving, making sure everyone gets a bit of melted provolone tucked inside the tender beef.
If your beef slices are on the thicker side, you can pound them gently with a meat mallet between pieces of plastic wrap to help them roll more easily and turn out extra tender. For smaller households, halve the recipe and bake it in an 8×8-inch glass dish, keeping the same baking time. If you like a slightly thicker gravy, remove the beef rolls to a plate after baking, then simmer the onion soup in a small saucepan on the stove for a few minutes to reduce before pouring it back over. You can also swap the provolone for another mild, good-melting cheese like mozzarella or Monterey Jack while still keeping the ingredient count at three. For make-ahead convenience, assemble the rolls and pour the soup over them earlier in the day, cover, and refrigerate; when suppertime comes, slide the dish into the oven and add 5–10 minutes to the baking time. Leftovers reheat nicely, especially if you slice the rolls and warm them gently in their gravy, and they make wonderful open-faced sandwiches on toast or a bun.
