Turn your oven to 400°F and line a large baking sheet with parchment paper. While that heats up, lay one sheet of thawed puff pastry on a lightly floured counter with the long side facing you. Keep the second sheet in the fridge so it doesn't get too soft while you work.
Brush the pastry with about half the mustard, leaving a small bare edge around the border. Sprinkle half the parmesan evenly over the top, then lay three slices of prosciutto across the surface. Don't overload it a thin, even layer rolls up much more neatly than a thick one.
Starting from the bottom, roll the pastry up tightly, jelly-roll style, and pinch the seam closed so it doesn't unravel. Repeat the same layering and rolling process with your second sheet of pastry, mustard, cheese, and prosciutto.
Pop both rolls into the freezer for about 5 minutes. This short chill firms up the butter in the pastry so your knife glides through cleanly instead of squishing the spiral shape.
Using a sharp knife, cut each roll into rounds about 1/2-inch thick. Lay them cut-side up on your prepared baking sheet, leaving a little space between each one so they have room to puff.
Whiskth the water and brush it over the tops of each pinwheel. Sprinkle with a pinch of salt, then bake for 12-15 minutes, until they're puffed, golden, and crisp on the edges.