Cut the chicken breast into small pieces, approximately 1/2 inch / 1 cm in size.
Finely dice the onion.
Chop the leek and rinse it thoroughly to remove any dirt or grit.
Prepare the remaining filling ingredients before beginning the cooking process.
For additional flavor, heat a large non-stick pan over medium-high heat with 1 to 2 tbsp vegetable oil.
Add the chicken to the hot pan in batches.
Sprinkle the chicken lightly with salt.
Cook the chicken for 30 to 60 seconds on one side only, allowing it to lightly caramelize without cooking through completely.
Transfer the partially seared chicken to a plate and set aside until needed.
In a saucepan, melt the butter over medium heat.
Once the butter is hot, add the diced onion and chopped leek.
Sauté until the onion becomes translucent and begins to color lightly.
Sprinkle the flour, paprika, and sea salt over the onion and leek mixture.
Stir well until the flour is fully incorporated and no lumps remain.
Cook the flour mixture for 2 to 4 minutes, stirring frequently.
Add the chicken, sour cream, and milk or chicken stock to the saucepan.
Stir until the filling is evenly combined.
Cook for 15 to 20 minutes, or until the chicken is fully cooked through and the filling has thickened.
Add the shredded Monterey Jack cheese and chopped chives.
Stir until the cheese has melted and the filling is creamy.
Simmer over low heat for an additional 5 minutes, if needed, until the filling reaches a thick, spoonable consistency.
Taste the filling and season with additional salt and pepper as needed.
Transfer the filling to a bowl or shallow dish.
Allow the filling to cool completely before assembling the pies.