Melt the butter in a large heavy-bottomed pot over medium heat.
Add the diced onions and sauté for 5 minutes until translucent and soft.
Incorporate the sliced mushrooms and minced garlic; cook for 10-12 minutes until the mushrooms have released their moisture and developed a golden-brown color.
Sprinkle the flour over the mushroom mixture and stir continuously for 2 minutes to toast the flour and prevent a raw taste.
Gradually pour in the stock while whisking constantly to ensure a smooth, lump-free consistency.
Add the dried thyme and bring the mixture to a gentle simmer for 10 minutes.
Reduce heat to low and stir in the heavy cream, heating through for 3-5 minutes without allowing the liquid to reach a boil.
Season with salt and black pepper to taste, then garnish with fresh parsley before serving.