Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Roll the pastry out flat on a lightly floured surface with a floured rolling pin. Brush off any excess flour. In a small bowl, combine the mustard with a splash of water to loosen. Brush the mustard over the entire top of the pastry.
In a small bowl, stir together the cheese, thyme and a few grinds of black pepper. Sprinkle the entire sheet of pastry with the cheese mixture. Gently roll the top of the pastry again to lightly embed the topping into the pastry. Cut into 2-inch squares and pinch one set of opposite corners together on each square or leave flat. Place on the prepared baking sheet.
Bake until the squares are puffed and golden brown, 10 to 14 minutes. Serve warm or at room temperature.