Preheat your oven to 400°F (200°C).

Place the peeled and chopped potatoes in a large pot of salted water. Bring to a boil and cook until tender, about 15 minutes.

Drain the potatoes and return them to the pot. Mash them with 2 tablespoons of butter, the milk, and half of the minced garlic until smooth. Season with salt and pepper to taste.

While the potatoes cook, melt the remaining 2 tablespoons of butter in a large skillet over medium heat.

Add the chopped onion and sauté until softened, about 5 minutes. Add the remaining garlic and cook for another minute.

Sprinkle the flour over the onions and stir constantly for 1 minute to cook off the raw flour taste.

Gradually whisk in the chicken broth, heavy cream, and dried thyme. Simmer for 2-3 minutes until the sauce slightly thickens.

Stir the shredded chicken into the sauce until well coated, then remove from heat.

Transfer the creamy chicken mixture into the bottom of a 9x13 inch baking dish.

Spread the mashed potatoes evenly over the top of the chicken layer.

Sprinkle the grated Gruyère cheese over the potatoes.

Bake for 20-25 minutes, or until the cheese is melted and golden brown. Garnish with fresh parsley before serving.