Clean the mushrooms with a damp paper towel. Slice them into even pieces. Do not soak them in water. Mushrooms act like sponges and will get soggy. Slicing them evenly ensures they cook at the same rate.

Melt the butter in your large pot over medium heat. Add the cleaned and sliced leeks. Cook them until they are soft and translucent. This should take about five minutes. Add the minced garlic and cook for one more minute.

Turn the heat up slightly and add the sliced mushrooms. Stir them occasionally so they do not stick. Let them release their moisture and turn golden brown. This browning process is where all the deep flavor comes from.

Sprinkle the flour over the cooked mushrooms and stir well. Cook the flour for two minutes to remove the raw taste. Pour in the white wine to deglaze the pot. Scrape the bottom to get the tasty brown bits.

Slowly pour in the stock while stirring constantly. Add the fresh thyme leaves, salt, and pepper. Bring the liquid to a gentle boil. Reduce the heat and let it simmer for about twenty minutes.

Use an immersion blender to puree the soup until smooth. If you like texture, leave a few mushroom chunks behind. Be careful when blending hot liquids. If using a standard blender, work in small batches.

Return the blended soup to the pot over low heat. Stir in the heavy cream slowly. Let the soup warm through but do not let it boil again. Boiling can cause the cream to separate or curdle.

Taste the soup one last time. Add more salt or pepper if needed. Ladle the hot soup into warm bowls. Garnish with fresh parsley or a few sautéed mushroom slices. Serve immediately to your hungry guests.