Sake no Saikyomiso-zuke Horenso Sosu (Miso-Marinated Salmon)

Tama-miso sauce:
2 Tbsp. sake
3½ ounces (about 5 Tbsp.) Saikyo miso (sweet white miso)
1 Tbsp. sugar
1 large egg yolk
About ⅛ tsp. usukuchi shoyu (light-colored soy sauce), preferably, or regular shoyu
1¼ pounds fresh salmon or cod fillets, skinned or not, cut into 4 pieces
5 tsp. salt
8 ounces Saikyo miso (sweet white miso)
¼ cup sake
¼ cup mirin
¼ cup dry white wine
¼ cup rice vinegar
1 Tbsp. minced shallot
3½ ounces spinach leaves, 4 medium leaves reserved
Vegetable oil, for frying
6 to 8 Tbsp. extra-virgin olive oil, to taste

First, make the tama-miso sauce. In a small cup, mix the sake with 2 tablespoons water. In a suribachi or other mortar, grind the miso, sugar, and egg yolk to a smooth paste. Add the rice wine and water mixture little by little, grinding all the time. Season to taste with shoyu. Have at hand a bowl half-filled with cold water and ice cubes. Transfer the sauce to the top of a double-boiler, and cook the sauce over simmering water, stirring constantly and thoroughly so you do not scramble the egg, until the sauce becomes thicker, about 6 to 8 minutes. Set the bowl of sauce in the bowl of cold water and ice cubes to cool. Tama-miso may be stored in the refrigerator, covered, for three days. Heat the sauce through before using it.
Salt the fish on both sides, and rest it on a steel rack set over the pan, for 1 hour in the refrigerator.
In a medium bowl, soften the miso by stirring in the sake and mirin. Spread one-third of the miso mixture in the bottom of a large pan in which the fish can fit without overlapping. Lay a tightly woven cotton cloth or two layers of cheesecloth over the miso in the pan. Wipe the salted salmon with a paper towel to remove the salt and the liquid exuded from the fish. Place all the salmon pieces on the cloth in the pan, and cover them with another tightly woven cotton cloth or two layers of cheese cloth. Spread the remaining miso mixture over the cloth, covering the surface completely. Wrap the entire pan with plastic wrap, and refrigerate it for 5 hours.
In a small saucepan, combine the dry white wine, komezu, and shallot. Bring the mixture to a boil over medium heat. Reduce the heat to very low, and cook the mixture until it is reduced to 1 tablespoon syrup.
In a large pot of boiling water, parboil the spinach, excluding the 4 leaves, 1 to 2 minutes. Cool the spinach in ice water, and drain the spinach well. In a food processor, purée the spinach. In a skillet, heat 1 inch oil over medium heat to 320°F. One at a time, add the 4 reserved spinach leaves to the oil, and cook them until they are bright green and translucent, 10 to 15 seconds. Transfer the spinach to paper towels to drain.
Lift the top cloth (or cloths) from the salmon, and remove the salmon from the marinade. Discard the marinade, or reserve it to use as a fish marinade one more time within 2 weeks, after heating it through and adding more miso and sake, or for making miso soup. If there is any miso residue on the fish, gently wipe it away with a paper towel. At this point you can refrigerate the fish, in a well-sealed plastic bag, for up to 3 days, or freeze it for a longer period.
Heat a broiler or grill, and the broiler pan or grill rack. With a pastry brush, lightly grease the pan or rack. Transfer the salmon to the pan or rack, and cook the salmon, turning once, until both sides are light golden. A 1-inch-thick salmon steak needs about 8 minutes’ total cooking. Marinated fish burns easily, so you may need to cover the fish with aluminum foil as it cooks.
In a small saucepan, combine 2 Tbsp tama-miso sauce with the reduced vinegar-wine syrup. Place the saucepan over low heat, and cook until the mixture is heated through. Transfer the mixture to a medium bowl, and add the spinach purée. Little by little, whisk in the olive oil. Serve the salmon with the spinach sauce underneath and garnished with the fried spinach leaves.