Preheat your oven to 375°F (190°C). Bring a medium pot of salted water to a boil, parboil the sliced potatoes for 4 minutes until slightly softened but not cooked through, then drain and set aside.

Heat the olive oil in a large ovenproof skillet or casserole dish over medium-high heat. Season the chicken pieces with salt and pepper, then sear them in the hot skillet until golden brown on all sides, about 5 to 6 minutes, and transfer to a plate.

In the same skillet, add the sliced mushrooms and diced onion. Cook for 6 to 8 minutes until the onions are soft and translucent and the mushrooms have released their moisture and turned golden brown.

Stir in the minced garlic and fresh thyme, cooking for 1 minute until fragrant. Sprinkle the flour over the vegetables, stirring constantly for 1 minute to coat them and cook out the raw flour taste.

Slowly pour in the chicken broth while whisking continuously to prevent lumps. Bring the mixture to a simmer and let it bubble for 3 minutes until the gravy thickens slightly, then stir the cooked chicken and any resting juices back into the skillet.

Arrange the parboiled potato slices on top of the chicken and mushroom mixture in overlapping concentric circles, completely covering the filling. Brush the melted butter evenly over the potatoes and season with a pinch of salt and pepper.

Cover the skillet with foil and bake for 30 minutes. Remove the foil and bake for an additional 20 to 25 minutes, or until the potatoes are tender and the edges are beautifully crispy and golden brown.

Remove from the oven and let the hotpot rest for 5 minutes before serving directly from the skillet alongside steamed green vegetables like peas or broccoli.