Put spinach in a mixing bowl and pour boiling water over to wilt it. Drain in a colander, rinse with cool water and squeeze completely dry. Chop the spinach as finely as you can and set aside.
preheat oven to 375 degrees. Season fillets with salt and pepper.
Line a roasting pan with a big piece of foil slightly longer than the fish, leaving ends hanging over. Smear middle section of foil lengthwise with 1/2 tablespoon soft butter and set fish on top. Smear top of fillets with remaining 1/2 tablespoon soft butter. Fold the sides of foil to the center and press against fish fillets. Twist both ends of foil to make a tight package. Bake for 30 minutes. Remove pan from oven and let fish rest 5 to 10 minutes, still in the foil package, while you make the sauce.
Put crème fraîche in a wide saucepan or skillet over medi- um high heat and bring to a simmer. Cook for a minute or so, until slightly reduced. Add cooked spinach, stirring to coat. Season with salt and pepper and turn heat to low. Quickly stir in 1 tablespoon chilled butter. mix well. Remove from heat. Stir in lemon zest, tarragon and chives.
Transfer fish to a warm serving platter. Carefully remove foil. (Fish should be cooked through but moist.) Pour any collected pan juices, then pour sauce over fish.