Melt butter & sauté mushrooms
In a large saucepan or Dutch oven over medium‑high heat, melt the butter. Add the sliced mushrooms and immediately season with salt and pepper. Stir frequently and sauté until the mushrooms release their moisture and then brown nicely—about 12 minutes total. You want good caramelization and for most of the liquid to evaporate.

Add onion and soft‑cook
Stir in the chopped onion. Cook about 3 minutes, stirring occasionally, until the onion softens and becomes translucent (but don’t let it burn).

Add garlic & flour
Add the minced garlic and cook for about 30 seconds until fragrant. Then sprinkle in the flour and stir thoroughly, coating the vegetables to make a roux‑like base.

Deglaze with sherry or wine
Pour in the dry sherry or white wine. Increase heat slightly so it bubbles. Let it cook until reduced by roughly half, scraping up the browned bits from the bottom of the pan with a spatula or spoon.

Add liquids, herbs & simmer
Pour in the milk and the stock (or water). Add the bay leaves and thyme sprigs. Stir to combine, bring to a bare simmer (not a boil), and let it cook gently for 20 minutes to allow flavors to meld.

Remove herbs & blend
Use tongs or a slotted spoon to fish out the bay leaves and thyme sprigs. Then blend the soup—ideally with an immersion blender right in the pot, or carefully in batches using a countertop blender. Aim for a smooth, silky consistency (or leave a bit of texture if you like some mushroom bits).

Finish & season
Taste and adjust with salt and fresh ground pepper. If you want brightness, squeeze in a little fresh lemon juice (a teaspoon or so). That last touch can elevate the overall flavor.

Serve with garnishes
Ladle into bowls and sprinkle with the minced fresh herbs. Finish with a light drizzle of extra‑virgin olive oil for shine and richness.