How to Make It
Set the fish on a platter. In a small bowl, combine 1/4 cup of salt with the sugar and pat it over both sides of the fillets. Let stand for 30 minutes. Rinse the fish and pat dry.
Meanwhile, in a small skillet, toast the walnuts over moderate heat for about 3 minutes, until golden. Transfer to a small bowl and let cool. Stir in the butter, juniper berries, orange zest, ham, lime juice and fennel pollen. Scrape the nut butter onto a sheet of wax paper and spread to a 3-by-6-inch rectangle; wrap in the paper. Refrigerate until firm.
Put the garlic in a small saucepan, add cold water to cover and bring to a boil. Drain. Repeat this process 3 times, reserving 2 tablespoons of the final water. Transfer the garlic and reserved water to a blender. Add 2 tablespoons of the olive oil and 1 tablespoon of the grapeseed oil and puree until smooth. Scrape the garlic puree into a bowl.
Add the watercress, arugula, parsley, basil and dill to the blender and pulse until finely chopped. With the machine on, pour in the remaining 1/4 cup each of olive oil and grapeseed oil and puree until smooth. Add the Parmigiano, lemon zest, lemon juice and the garlic puree and pulse to combine. Season the pistou with salt and pepper and transfer to a bowl.
Light a grill or preheat a grill pan and brush it with olive oil. Brush the fish with olive oil and grill over moderately high heat, turning once, until cooked through and lightly charred, 6 minutes. Spoon the pistou onto 4 plates and top with the fish. Unwrap the butter and cut it into 4 rectangles. Place the butter on the fish and serve.