Start by preparing the chicken filling. In a medium saucepan, melt the butter over medium heat. Once the butter is melted and bubbling, add the chopped onions and sauté them until they become translucent. This will take about 3-4 minutes.

Next, sprinkle in the flour and stir well to form a smooth paste. Allow the flour to cook for 2-3 minutes, stirring frequently to avoid burning. This step ensures the flour is cooked through and helps thicken the filling.

Add the leeks, chicken, sour cream, and half and half to the pan. Season with salt and pepper, then stir everything together. Cook the mixture for about 20 minutes, or until the chicken is fully cooked and tender.

Afterward, stir in the paprika, chopped chives, and shredded Monterey Jack cheese. Allow the filling to simmer for an additional 10-15 minutes, ensuring all the flavors meld together. Once done, remove the pan from the heat and set the filling aside to cool.

While the filling cools, preheat your oven to 375°F (200°C) and line your baking sheets with parchment paper.

Lightly dust a clean surface with flour and roll out the puff pastry to about 2/8 inch in thickness. If you're using puff pastry sheets, make sure to roll them out evenly. Using a 3 to 3.5-inch round cutter, cut as many circles as possible from the rolled-out dough. You should be able to make around 40-45 circles, depending on the size of your cutter.

Once you have your pastry circles, brush the edges of one circle with egg wash. Spoon a small amount of cooled chicken filling into the center of the circle. Be careful not to overfill it. Top with another pastry circle and press down the edges to seal.

Use a fork to crimp the edges and ensure they are sealed tightly. For a clean finish, you can also use the round cutter to trim off any excess pastry, giving your pastries a neat and uniform look.

Once assembled, place the pastries on the prepared baking sheet, leaving about 1 inch of space between them. Brush the tops of the pastries with the remaining egg wash and make two small slits on top to allow steam to escape during baking.

Bake your chicken pastries in the preheated oven for about 20 minutes, or until the tops are golden brown and puffed up. When done, remove them from the oven and let them cool for a few minutes before serving. Enjoy these warm, delicious pastries!