Simmer all ingredients under ‘Stock Ingredients’ together for 1 1/2 hours. Strain and chill.
Preparing the Soup:
Soften butter in pot and add flour. Cook this roux until it scrapes pot clean (approx. 5 minutes)
Add 1/4 of the stock (cold) to roux and whisk until smooth. Continue adding stock slowly, maintaining a velvety consistency until all stock is integrated. Cook 10 minutes and reserve this mushroom veloute.
Using the grating attachment on a food processor, grate the button mushrooms, shiitake mushrooms and onion to a medium coarseness.
In a large skillet saute grated mushrooms and onions and minced garlic until all moisture is removed.
Add mushroom veloute to mushroom mixture and simmer for 10 minutes. Turn off heat. Add cream and chopped tarragon.
For an added touch, top with a small amount of white truffle oil and chopped raw enoki mushrooms.