In a skillet melt the butter over medium-high heat and sauté the potatoes, onions, carrots and garlic until golden. Add the peas and simmer for 1 minute. Add the chicken stock and simmer for 20 minutes until the vegetables are tender.
Whisk together the milk and cornstarch until smooth then add to the vegetable mix and simmer until thickened. Add the thyme and parsley to the vegetables and set aside.
Cut the chicken breast pieces into 1 inch square pieces. Season the chicken with salt and pepper, then in a large skillet heat the oil over high heat and sear the chicken until golden and cooked through. Add the chicken to the vegetable mixture.
Topping: Boil potatoes until soft. Drain, then add remaining ingredients and mash well. Season with salt and pepper.
To assemble: use a greased rectangular baking dish, or for individual portions fill 6 ramekins with 1 cup of filling and top with the mashed potatoes. Bake at 350°F for 30 minutes.