1. Cook the mushrooms according to package directions, then drain thoroughly.

2. Microwave the spinach in a covered bowl for 2 minutes on high. Stir, then heat for another 1½ minutes. Press out as much moisture as possible.

3. Finely chop both the mushrooms and spinach, then combine in a large bowl and season to taste.

4. On a lightly floured surface, unroll the puff pastry and roll it into a rectangle about 16 x 12 inches.

5. Spread the spinach-mushroom mixture evenly over the pastry, leaving a ½-inch border. Sprinkle with the grated cheese.

6. Starting from one short side, roll the pastry tightly into a log. Chill for 10 minutes if time allows.

7. Preheat the oven to 400°F.

8. Slice the chilled pastry log into 24 thin rounds. Arrange the pieces on a large parchment-lined baking sheet.

9. Brush the tops lightly with beaten egg and bake for 20–25 minutes, or until crisp and golden.

10. Serve warm with salsa or ketchup if desired.