Chicken Mushroom Strudel

Name: Chicken Mushroom Strudel
Prep time: 30 min
Total time: 30 min
Cuisine: German
Ingredients: 1 roasted chicken; 2 tablespoons butter; 1.25 pounds fresh mushrooms, thickly sliced; 1.5 teaspoons kosher salt, divided, or to taste; 0.667 cup diced yellow onion; 0.25 teaspoon freshly ground black pepper; 0.25 teaspoon ground cumin; 0.25 teaspoon smoked paprika; 1 pinch cayenne pepper; 1 cup heavy cream; 0.25 cup chopped fresh tarragon; 1 lemon, zested and juiced; 0.5 cup grated Gruyere cheese; 2 (14 ounce) packages frozen puff pastry, thawed; 1 large egg for egg wash