If you don't have porcini mushrooms, substitute cremini or white button, the dish will still be great with tons of flavor.
Season the chicken breasts with salt and pepper.
Heat the olive oil in a medium pot over high heat. Sauté the mushrooms for 1 minute. Add the leeks, carrots and shallots and sauté for 3 minutes.
Add the wine, mustard, bay leaf and sprig of tarragon. Bring to a low boil and add the chicken breasts. Reduce the heat and simmer, covered, for 10 to 15 minutes, flipping the chicken a couple of times.
Transfer the chicken breasts to a plate and tent loosely with foil. Increase the heat to medium-high and cook the sauce until it starts to thicken. Then whisk in the butter, the remaining 2 teaspoons tarragon, the parsley and dill. Taste and adjust the seasoning.
Serve each chicken breast with a generous amount of sauce, 1 tablespoon of the crème fraîche and a dash of lemon zest. Garnish with 2 sprigs of dill and lemon wheels.