Lay a slice of roast beef flat on a cutting board. If the slices are very large, cut them in half so they’re easier to roll into tight spirals. Pat them dry with a paper towel if they’re very wet so the filling sticks better.
Place a half slice of provolone on top of the roast beef, lining it up near one edge so it will roll neatly. If your roast beef is smaller, you can trim the cheese a bit so it doesn’t hang out too far.
Spread a thin layer of the mustard mixture over the provolone, going almost to the edges but not quite. You want enough to taste tangy and salty, but not so much that it squishes out when you roll.
Starting at the cheese-lined edge, roll the roast beef up tightly into a log, pressing gently as you go so the cheese and mustard stay tucked inside. Repeat with remaining roast beef, provolone, and mustard mixture until everything is used.
Using a sharp knife, slice each roll into bite-size spirals, about 1 to 1 1/2 inches thick. Try to keep them as even as possible so they bake at the same rate and look like neat little pinwheels.
Arrange the spirals cut side up in the greased glass casserole dishes, packing them in snugly so they help hold each other together as they bake. The tighter they’re nestled, the better the seams will melt and seal.
In a tiny bowl, stir together the finishing mustard and 1 teaspoon of water to make a loose drizzle. Using a spoon, lightly drizzle it over the tops of the spirals for a shiny, tangy finish.
Bake in the preheated oven for 12 to 15 minutes, or until the provolone is melted, the edges of the roast beef are just starting to crisp, and the seams look melty and sealed. Keep an eye on them so the cheese doesn’t burn.
