How to Make It
Season the fish all over with salt and pepper. Fill each cavity with 1 sprig of sage and 1 slice of lemon. Place 2 sprigs of sage and 1 slice of lemon on both sides of each fish.
Place a sheet of wax paper on a work surface. Arrange 6 to 8 slices of the pancetta in 2 overlapping rows to form a 6-by-8-inch rectangle. Set 1 fish on the bottom edge of the pancetta. Using the wax paper as a guide, tightly roll up the fish in the pancetta. Carefully peel off the wax paper. Brush the fish with canola oil and transfer to a rimmed baking sheet. Repeat with the remaining fish and pancetta and more oil.
Preheat a grill or a broiler with a rack in the top third of the oven. Grill or broil the fish, flipping halfway through, until cooked through, about 6 minutes per side. Season with pepper.
Meanwhile, in a small skillet, melt the butter. Cook over moderately high heat, stirring occasionally, until browned, 2 to 3 minutes. Stir in the shallot, parsley, tarragon, scallion, mint and tomato paste. Cook, stirring, for 20 seconds. Add the wine, bring the sauce to a boil and cook for 1 minute. Add 2 tablespoons of the cream and cook until the sauce thickens, about 1 minute. Scrape the sauce into a small heatproof bowl and stir in the lemon juice. Season with salt.
In a large bowl, combine the mustard with the vinegar. Slowly drizzle in the hazelnut and peanut oils, whisking constantly, until incorporated. Whisk in the remaining 2 tablespoons of cream. Season with salt and pepper. Add the watercress and chives and toss to coat. Serve with the fish and butter sauce.