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1
Preheat oven to 375u00b0. Dice enough cucumber to make 3/4 cup. Cut remaining cucumber in half lengthwise. Scrape out any coarse seeds with a small spoon and discard. Thinly slice cucumber crosswise.
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2
Set salmon skin side down on a work surface. Cut fish down to the skin, but not through it, to divide into 4 portions.
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3
Line a 9- by 13-in. baking dish with foil. Put salmon skin side down in dish. Melt 1 tbsp. butter and mix with lemon juice, 1/4 tsp. salt, and the pepper; spoon over fish and into crevices.
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4
Bake salmon until barely opaque but still moist in center of thickest part, 10 to 13 minutes. Remove from oven; keep warm.
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5
While salmon bakes, combine shallots, chopped tarragon, and the vinegar in a 10-in. frying pan. Boil over high heat, stirring often, until vinegar evaporates but shallots still look wet, 4 to 5 minutes. Add 1 tbsp. butter, remove from heat, and stir until it melts. Stir in diced cucumber; set aside.
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6
Transfer cooked fish to a warm platter or plates, using a wide spatula to lift it from skin. Return cucumber mixture to high heat and cook, stirring often, until slightly translucent and hot, 1 to 2 minutes. Add remaining 3 tbsp. butter and 1/4 tsp. salt. Stir just until butter melts and sauce looks creamy. Remove from heat and stir in sliced cucumber and parsley.
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7
Spoon cucumber sauce over salmon and scatter tarragon leaves on top. Serve immediately so colors stay bright, and season to taste with more salt if you like.
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8
*Ask your fishmonger to remove bones, or do it yourself: Run a finger over flesh to locate bones, then pull them out with fish tweezers (see how at sunset.com/salmonvideo).
