In a large saucepan, melt 1 Tbsp. of the butter over medium-high heat. Add the onions and cook, stirring, until soft, about 3 minutes.
Add the mushrooms, thyme, salt, and pepper and the second 1 Tbsp. of butter. Cook, stirring regularly, until the mushrooms start to brown, about 5 minutes.
Add the last 1 Tbsp. of butter and stir to melt. Add the flour and cook, stirring constantly for about 30 seconds to a minute. Add the stock and stir while returning the mixture to a boil. Reduce the heat to medium-low and simmer uncovered, stirring occasionally, for 10 minutes.
*At this point, if you like a thicker soup, you can remove a few of the mushrooms with a bit of soup to puree and return to the pot or simply pulse a few seconds with an immersion blender. Neither is necessary if you'd rather leave it with all whole mushroom pieces.
Add the cream and cook for a further minute or two to heat. Taste and adjust seasoning, as necessary. Enjoy!
Be sure to read the "Cook's Notes" in the original post, for more tips, options, substitutions and variations for this recipe!