Heat a large heavy-bottomed soup pot over medium heat and add the olive oil and butter.
When the butter has melted, add the diced onion, garlic and thyme.
Saute just until the onion softens and turns transluscent.
Add the mushrooms and stir well, letting the mushrooms soften and release their juices but cooking them only until they still keep their shape (about 5-10 minutes).
Add the salt and pepper, stirring well.
Add the white wine and scrape any brown bits off the bottom of the pot while the wine cooks off.
Add the stock, stir well, and cover the pot, turning the heat to medium low. Let the soup simmer for about 15 minutes.
Remove the lid and turn off the heat under the pot.
Whisk together the heavy cream and the flour until the flour is incorporated with no lumps.
Slowly add the cream to the hot soup (NOT while it is boiling or simmering) and stir well.
Let the soup sit, covered, for about 4-5 minutes before serving.