Preheat the oven to 375 degrees. Heat the butter in a large skillet over medium heat. Saute the leeks for one minute, add the carrots, and saute for about two minutes more. Add the radishes and asparagus and saute for one minute.

Sprinkle flour over the vegetables and stir until evenly distributed. Add the chicken stock, turn the heat to high, and stir until it comes to a boil and starts to thicken. Season with salt and pepper, then turn off the heat. Stir in the mascarpone until it's fully melted and incorporated. Gently fold in the chicken, frozen peas, lemon zest, lemon juice and chives. Taste and adjust seasoning as needed.

Divide the filling amongst 6 small ramekins. Dust a work surface with flour and roll out the puff pastry to even out any folds and make it slightly thinner. Cut the pastry into 6 squares and cut a slit in the center of each. Place the pastry squares over each ramekin, pressing down gently around the lip and letting the excess fall over the sides. Brush the tops lightly with egg wash, then place the ramekins on a large sheet pan. Alternatively, you can pour the filling into a large casserole dish and place the whole sheet of pastry on top.

Bake for about 30 minutes, or until the pastry is puffed and golden brown. Allow the pot pies to cool for 10 minutes before serving.