1. Peel potatoes and with a very sharp slicer or number one slicing blade of your food processor, cut lengthwise into paper-thin slices.
2. Arrange overlapping slices to form a rectangle on a board, to form a rectangle as wide as one of the salmon pieces is long (about 6 ½-inches) and long enough to wrap around it (about 8-inches). You will need about 2 overlapping rows of about 4 overlapping slices.
3. Season salmon with salt and pepper and if you like, the optional Sweet Paprika or Ancho Chile, on both sides.
4. Place salmon crosswise over potato “seam in center of rectangle.” Lift potato slices around salmon to enclose, overlapping slightly. Wrap remaining salmon pieces in potato slices in the same manner.
5. In large skillet heat 1/3-inch oil over moderate heat until hot but not smoking. Carefully slide the 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.
6. In a skillet cook 2-3 garlic cloves, peeled, lightly smashed, in 4 teaspoons butter and 2 teaspoons extra virgin olive oil, slowly until golden brown. Add a slice of lemon with the garlic if you like. Then add spinach, stirring until just wilted, about one minute.
7. Season spinach with salt and pepper, taste for seasoning. A pinch of freshly ground nutmeg is nice. Place in a strainer to drain. Remove garlic and discard or enjoy.
8. In a cleaned skillet heat lemon juice, water and mustard over moderately low heat, add Serrano chilies if you like, and stir.
9. Add remaining butter all at once and heat, swirling constantly, just until sauce is creamy and butter is incorporated. Sauce must not get hot enough to separate.
10. Remove skillet from heat and season with salt, pepper and if you like, Smoky Sweet Paprika or Ancho Chile.
11. To keep warm, place skillet in a hot water bath. Stir sauce before serving. If sauce gets too hot, add a teaspoon of cold butter.
12. Mound spinach in the center of four heated plates and top with salmon. Pour mustard sauce around spinach.