Peel potato and with a very sharp slicer cut lengthwise into paper-thin slices.
On a work surface arrange overlapping potato slices to form a rectangle as wide as one of the salmon pieces is long (about 6 1/2 inches) and long enough to wrap around it (about 8 inches). You will need about 2 overlapping rows of about 4 overlapping slices. Season salmon with salt and pepper and place crosswise over potato "seam" in center of rectangle. Lift potato slices around salmon to enclose, overlapping slightly. Wrap remaining salmon piece in potato slices in same manner.
In a large skillet heat 1/3-inch oil over moderate heat until hot but not smoking. Carefully slide wrapped salmon with a spatula into oil and fry 3 minutes, or until undersides are golden. Carefully turn salmon over with tongs or 2 spatulas and fry until potato is golden and salmon is just cooked through, about 3 minutes more. Transfer wrapped salmon to paper towels to drain.
In a skillet cook spinach in 1 teaspoon butter over moderate heat, stirring, until just wilted, about 1 minute. Season spinach with salt and pepper and transfer to paper towels to drain.
In cleaned skillet heat lemon juice, water, and mustard over moderately low heat, stirring. Add remaining 3/4 stick butter all at once and heat, swirling skillet constantly, just until sauce is creamy and butter is incorporated. Sauce must not get hot enough to separate. Remove skillet from heat and season sauce with salt and pepper.
Mound spinach in center of 2 heated plates and top with salmon. Pour mustard sauce around spinach.
Recipe courtesy of Gourmet Magazine