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1
Put spinach in a mixing bowl and pour boiling water over to wilt it.
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2
Drain in a colander, rinse with cool water and squeeze completely dry.
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3
Chop the spinach as finely as you can and set aside.
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4
Heat oven to 375 degrees.
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5
Rinse fish and pat dry.
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6
Season fish inside and out with salt and pepper.
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7
Put a few tarragon springs in the belly cavity.
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8
Line a roasting pan with a big piece of foil slightly longer than the fish, leaving ends hanging over.
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9
Smear middle section of foil lengthwise with 1 tablespoon soft butter and set fish on top.
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10
Smear top of fish with remaining tablespoon soft butter.
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11
Fold the sides of foil to the center and press against fish.
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12
Twist both ends of foil to make a tight package.
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13
Bake for 30 minutes.
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14
Remove pan from oven and let fish rest 5 to 10 minutes, still in the foil package, while you make the sauce.
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15
Put creme fraiche in a wide saucepan or skillet over medium high heat and bring to a simmer.
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16
Cook for a minute or so, until slightly reduced.
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17
Add cooked spinach, stirring to coat.
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18
Season with salt and pepper and turn heat to low.
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19
Quickly stir in 1 tablespoon chilled butter at a time.
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20
Each spoonful should be just melted before adding the next, to make a creamy sauce.
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21
Remove from heat.
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22
Stir in lemon zest, tarragon and chives.
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23
Transfer fish to a warm serving platter.
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24
Carefully remove foil.
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25
(Fish should be cooked through but moist.)
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26
Peel away and discard skin from top of fish.
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27
Pour any collected pan juices into the sauce, then spoon sauce over fish.
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28
Serve with boiled new potatoes if desired.
