Cheese Curds and Sausage in Puff Pastry

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Ingredients:

  • 4 ounces kielbasa, sliced
  • 1 sheet frozen puff pastry (half a 17- ounce plan), defrosted
  • 2 teaspoons spicy brown mustard, plus more for serving
  • 6 ounces cheese curds (or part-skim mozzarella, cut into little pieces)
  • 1 big egg, gently beaten
  • Flaky salt