Preheat oven to 400ºF.
Place a large skillet over medium-high heat with 1 turns of the pan of EVOO, about 1 tablespoons. Season the fish with salt and pepper, and sear in the hot pan until golden brown. Remove and reserve.
Return the pan to medium-high heat and add 1 more turn of the pan of EVOO, about 1 tablespoon. Season the fish with salt and pepper, and sear fish a minute on each side. Remove and reserve. Add 1 more turn of the pan of EVOO, about 1 tablespoon. Add the mushrooms and leeks to the pan, and cook until tender, 6-7 minutes. Season mushrooms with salt and pepper then add tarragon, cream and Dijon to the skillet. Bring up to a bubble, then reduce the heat to medium and simmer until thickened, 3 minutes. Remove from the heat and let cool.