Dice the mushroom stems, and slice the caps ~ 1/4"
In a medium stock pot / sauce pan over low heat, add 2 tbsp. butter and foam out, add the leeks and onion, until translucent ~10-15 minutes, add garlic and sweat until fragrant, ~3 minutes
In small stock pot / sauce pan over medium heat, add the broth and diced mushroom stems, simmer gently until the stems soften. ~ 30 minutes.
Add the leeks, onion and garlic mixture to the stock, Return the stock pot to medium heat. (OPTIONAL) use a stick blender to puree the stock.
Add 4 tbsp of butter to the stock pot and foam out, add the mushroom slices and sauté gently until browned and tender. Add the flour and cook until browned, ~ 1 minute. Scrape the fond from the bottom of the stock pot. Add the half and half, cream, and herbs. Simmer to thicken, ~ 2-3 minutes.
Add the stock to the roux and mushroom mixture, and stir gently, simmer to thicken ~ 3 minutes, DO NOT BOIL. Season to taste with salt and pepper.