Preheat the oven to 200°C (180°C fan).
Bring a large pot of salted water to a boil. Add the potato chunks and cook until tender, about 15-20 minutes.
Drain the potatoes and return them to the pot. Add the butter and milk and mash until smooth. Season with salt and pepper.
In a large skillet, heat the olive oil over medium heat. Add the onion and garlic and cook for 2-3 minutes until softened.
Add the chicken and bacon and cook for 5-7 minutes until the chicken is cooked through.
Sprinkle the flour over the chicken mixture and stir to coat. Gradually pour in the chicken stock, stirring constantly, until the mixture thickens.
Stir in the thyme and rosemary and season with salt and pepper.
Transfer the chicken mixture to a 9-inch pie dish. Top with the mashed potatoes, spreading them evenly.
Bake for 25-30 minutes, or until the potatoes are golden brown.
Serve hot.
