Heat olive oil in a pan on medium-high heat and saute the sliced mushrooms for 5 minutes.
Add the minced garlic and stir in for about 1 minute.
Then mix in the chopped leeks, chicken stock and salt & pepper and lower the heat to low for about 5 minutes, until liquids are reduced by half.
During this time, boil a pot of water and add your past of choice.
Once the sauce liquids are reduced add heavy cream and tarragon again let it simmer on low heat for 3 - 4 minutes.