In a large pot, heat the olive oil and 2 T butter over medium high heat.
Add the mushrooms and 1 tsp salt and saute until golden brown, about 10 minutes.
Whisk in the remaining 2 T butter and flour; cook for 1-2 minutes to cook off the raw flour taste. Deglaze the pot with the sherry.
Slowly add the milk and cream, whisking until smooth. Add the thyme sprigs and bring the mixture to a simmer.
Allow to cook until creamy and thickened. Season with pepper and additional salt, if desired. Remove any thyme stems and serve.