Melt ¼ cup of the unsalted butter in a skillet. Sauté the button mushrooms for 5-7 minutes until browned. Add minced garlic and cook for 1 more minute. Set aside.

In the same skillet, melt another ¼ cup butter. Sear the chicken thighs for 4-5 minutes on each side until golden. Remove and chop into pieces.

Cook bacon in the skillet for 3-4 minutes until crispy. Add onion wedges and sauté for 4-5 minutes. Stir in remaining garlic and cook for 1 more minute.

Pour Guinness into the skillet, scraping browned bits. Simmer and reduce by 75% for 3-4 minutes. Add flour and remaining butter to form a roux. Gradually stir in beef stock and tomato paste until smooth.

Return mushrooms and chicken to the sauce, simmer for 10-15 minutes until thickened and creamy. Preheat oven to 200°C (390°F).

Transfer filling to a greased baking dish. Prepare Duchess Potato topping by piping or spreading it on top.

Drizzle melted butter over topping and sprinkle with Parmesan cheese. Bake for about 30 minutes until golden and crispy.