Melt butter in pan, add oil and sauté shallots and garlic over medium heat until just translucent (approx. 3 min). In a separate bowl, whisk together 1/2 of the stock with the starch, pepper, tarragon, and Mushroom Powder. Once the shallots and garlic are translucent, add the stock mixture as well as the remaining stock and bring to a boil. Once boiling, turn the heat off, and add the cream. Puree with an immersion blender to combine.