In a large heavy-bottom pot or dutch oven set over medium heat, melt 2 tbsp butter. Add garlic and saute quickly, then stir in oyster mushrooms and salt. Cook for about 5 minutes, until mushrooms are softened and lightly browned then remove from pot and set aside.
Add remaining 2 tbsp butter to pot along with onions and celery and saute until veggies are softened and onions are translucent, about 5 minutes.
Add potatoes, thyme, Old Bay seasoning and bay leaf and cook another minute then stir in instant potato flakes and stock. Heat until thickened then stir in milk and whipping cream. Bring to a simmer and cook until potatoes are just tender then add sauteed mushrooms back to the pot and cook for another 5 minutes.
Season to taste with salt and freshly cracked black pepper and transfer to 4 soup bowls. Garnish with chopped parsley and serve with crackers or sliced baguette.
