Preheat oven to 375 degrees F. Grease a 10-inch squared or oval casserole dish, set aside.

Place the diced potatoes in a medium pot and fill with cold water. Bring to a boil and cook until the potatoes are fork tender, about 15 to 20 minutes.

In the meantime, set a large pot over medium-high heat with 2 tablespoons butter. Once melted, add in the chicken and cook, stirring often, until browned and no longer pink, about 8 to 10 minutes. Using a slotted spoon, transfer the chicken to a plate and set aside. Enjoy!

Add a tablespoon of butter to the skillet and once melted, add in the onion, garlic, carrots, celery and leeks and cook until the vegetables have softened, about 5 minutes. Stir in the flour and cook for about 30 seconds. Stir in the chicken stock and 2 cups milk. Season with 2 teaspoons salt, 1 teaspoon pepper, bay leaf, thyme, and parsley; stir in the peas and the cooked chicken. Reduce heat to low and cook, stirring often, until the sauce has thickened, about 15 minutes.

In the meantime, drain the cooked potatoes and return to the pot they were cooked in. Add the remaining 4 tablespoons butter, 1 cup milk, 1 teaspoon salt, 1 teaspoon black pepper, and garlic powder. Mash with a potato masher until smooth. Give the potatoes a taste and adjust seasoning accordingly.

Transfer the pot pie mixture into the prepared dish. Then top with mashed potatoes, you can do this by spooning it on top and spreading it out or by transferring to a piping bag, fitted with a star tip, and piping it out in rows.

Place the dish on a baking sheet and bake until the bubbly and the top is golden brown, about 20 to 25 minutes. Remove from the oven and garnish with fresh thyme before serving. Enjoy!