For the anchovy butter: Process all the ingredients in a food processor or mini chopper until smooth and creamy. If not serving immediately, refrigerate.

For the sole: Beat the eggs in a shallow bowl until smooth and well combined. Pat the fish fillets with paper towels to dry them thoroughly. Sprinkle both sides with the salt. Put the fried onions in a food processor and process until smooth and powdery. Transfer the onion mixture to a large plate or cookie sheet.

Heat the oil in a large nonstick skillet or divide the oil between two slightly smaller skillets. Dip the fish fillets in the eggs and then into the powdered onion. Arrange in one layer in the skillet(s) and cook for 11/2 to 2 minutes on each side, until well browned and barely cooked in the center, turning carefully with a large spatula. Transfer to warm plates and serve each with a lemon wedge and a spoonful of anchovy butter on the side.

Heat 2 tablespoons of the olive oil in a large skillet over high heat. Add the onion and cook for 1 minute. Add the spinach, pushing it down into the skillet, and 1/4 teaspoon each of the salt and pepper. Cover and cook over medium heat for about 11/2 minutes, until the spinach is wilted. Remove the lid and cook, uncovered, until the liquid from the spinach has evaporated. Transfer to a plate and let cool.

Trim the tenderloin of any fat and silverskin. To butterfly the tenderloin for stuffing, lay it flat on the cutting board so one end is close to you and the other end is near the top of the board. Holding your knife so the blade is parallel to the board, cut through the long side of the tenderloin, stopping when you are about 1/2 inch from the other side. Turn the tenderloin so the uncut side is closest to you and make another parallel cut below the first one, again stopping about 1/2 inch before you reach the other side. Open up the butterflied tenderloin and pound it a little to extend it to about 12 inches long by 7 inches wide.

Preheat the oven to 350 degrees. Arrange half the spinach mixture down the center of the butterflied tenderloin and top with the cheese. Add the rest of the spinach, fold in the sides, and roll the tenderloin back and forth to evenly distribute and encase the filling. Wrap 2 strips of aluminum foil, each 1 to 2 inches wide, around the tenderloin to secure the stuffing inside.

Heat the remaining 2 tablespoons oil in a large ovenproof nonstick skillet. Sprinkle the outside of the tenderloin with 1/4 teaspoon each of the salt and pepper. Place the tenderloin carefully in the skillet and brown it, turning occasionally, for about 5 minutes. Carefully remove the foil strips from the tenderloin and bake in the oven for 10 minutes, when it will be slightly pink in the center. Transfer the tenderloin to a plate, cover, and keep warm in the oven while you prepare the tomatoes (the pork will continue to cook as it sits).

Add the tomatoes and the remaining 1/4 teaspoon each salt and pepper to the skillet in which you browned the tenderloin and sauté over high heat for 11/2 to 2 minutes, until just softened. Divide among four warm plates.

Slice the tenderloin crosswise into 8 medallions and arrange 2 slices in the middle of the tomatoes on each plate. Serve.