Heat 2 tablespoons butter in a pot over medium-high heat.
4 tbsp butter
When the butter has melted, add mushrooms to the pot. Cook, stirring occasionally, until dark golden brown, about 5 minutes.
Season with salt and pepper, then remove them from the pot and reserve.
1 tsp sea salt, 1/2 tsp black pepper
In the same pot, melt the remaining 2 tablespoons butter. Reduce the heat to medium and, when the butter has melted, add the garlic, celery, thyme and shallots. Cook, stirring frequently, until the vegetables are soft, 5 to 6 minutes.
2 sprigs fresh thyme
Sprinkle the vegetables with the flour and stir to combine. Season with salt and pepper, then cook, stirring constantly, for 3 minutes, allowing the flour to lightly toast.
Add the dry white wine to the vegetable mixture.
3/4 cup dry white wine
Deglaze with the wine while scraping up any browned bits.
Slowly add the hot stock while stirring to reduce lumps.
Bring to a simmer and cook, stirring occasionally, until slightly reduced and thickened, about 20 minutes.
Add the cream and bring back to a simmer.
1 cup heavy cream
Add the mushrooms and parsley. Taste and adjust the seasonings if necessary.
1/2 bunch flat-leaf parsley
Serve hot!