Chicken Shepard's Pie

This is another recipe that I have on a card and don't know where it's from I think it's from the Gluten Free Club. I use frozen veggies to make it more budget friendly and also use gf bullion. If you aren't gf then use whatever you want.
I love this but I, as usual, just eyeball measurements for spices and do it to taste. I use one big casserole dish and prepare the potatoes ahead of time.
Filling:
2 Tbsp unsalted butter
2 Yukon Gold potatoes, diced
2 onions, diced
1 carrot, diced
2 tsp minced garlic
1 cup peas
1 1/2 cups chicken stock
1 cup milk
1/2 cup cornstarch
1 Tsp thyme
2 Tsp Italian parsley
salt and freshly ground black pepper
4  boneless chicken breasts
1 Tbsp vegetable oil

TOPPING
5 Yukon Gold potatoes, diced
3/4 cups 35% cream
1/4 cup unsalted butter
salt and freshly ground black pepper
Directions:
Preheat oven to 350°F.

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 peices. Season the chicken with salt and pepper. 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
Put the potatoes in a large pot, cover with water and season with salt. Over high heat bring to a boil and simmer until tender. Drain the potatoes and let them sit for 5 minutes to steam dry. Put the potatoes through a food mill or mash with an electric mixer. Warm the cream and stir into the potatoes. Mix in the butter. Season to taste.

To assemble the pot pies fill 6 ramekins with 1 cup (250 mL) of filling each and top with the mashed potatoes or place mixture in a greased 9x13 pan and spread the topping evenly over the top. Bake for 30 minutes.