Spring Chicken Pot Pie

Name: Spring Chicken Pot Pie
Ingredients: 2 tablespoons butter; 1 large leek (white and light green parts diced (about 1 cup)); 1 ½ cups carrots (diced); 1 cup radishes (diced); 1 cup asparagus (diced); 3 tablespoons flour; 1 cup homemade or low sodium chicken stock*; 2 teaspoons salt (plus more to taste*); freshly cracked black pepper (to taste); 8 ounces mascarpone cheese; 2 cups diced or shredded chicken; ¾ cup frozen peas; 2 teaspoons lemon zest; 2 teaspoons lemon juice; 2 tablespoons minced chives; 1 sheet all-butter puff pastry (such as Darfour); flour (for rolling); 1 egg (whisked together with 1 teaspoon water)