Bring a large pot of water to a boil. Warm the sauce in a saucepan.
Add salt to the boiling water and blanch the spinach for 1 minute. Lift out the leaves with a skimmer or large strainer, shaking hard to release excess moisture. Set the strainer over a saucepan half-filled with hot water to keep warm.
Break the lasagna noodles in half crosswise. Cook the noodles until al dente. Drain.
While the noodles are cooking, heat the ricotta in a saucepan. Stir constantly over low heat until warm. Do not overheat. Stir in the mozzarella until almost melted. Do not overheat. Remove from heat. Sprinkle generously with pepper and nutmeg.
Line up 4 wide-rimmed soup bowls. Working quickly, assemble the lasagna as follows: Using half of the sauce, spoon a pool of hot sauce in the bottom of each bowl. Top with 2pieces of lasagna noodles set side by side. Arrange a single layer of spinach over the noodles. Top with cheese, dividing it evenly among the plates. Top with a second layer of lasagna noodles. Top with the remaining sauce, dividing it evenly. Sprinkle lightly with grated cheese and serve at once.