Preheat oven to 350° F. Lightly grease a small baking dish.
Heat a saute pan over medium-high heat. Once heated, pour in the olive oil and add the onions and garlic. Cook for 2 minutes until onions become translucent and softened.
Lower the heat to medium and combine the cooked chicken, peas, carrots, potatoes, salt and pepper in the pan. Mix everything together thoroughly.
Pour the chicken stock, cover the pan and let the mixture cook for 5 minutes, stirring occasionally, until it is heated through.
Add the heavy cream and mix everything together well. Let it boil and turn off the heat.
Pour the chicken filling onto the baking dish. Top with rolled-out pie crust dough and gently stretch to cover the filling over the baking dish, tucking the edges well. With a sharp knife, make 4 small cuts in the dough.
Brush the dough with milk, you don't need to use it all.
Bake for 20-30 minutes, until the top crust is golden brown and fully cooked.
Remove from the oven, let it rest for a few minutes before cutting.