Preheat the oven to 180 ºC (350 ºF). In a saucepan, soften the spinach in the olive oil. Season with salt and pepper. Set aside.

In a saucepan over medium heat, melt the butter. Add the flour and cook for 1 minute, stirring constantly. Add the milk and stir until it thickens. Add the cheese and nutmeg. Stir until the cheese has melted. Season with salt and pepper. Set aside.

Cut the lasagna noodles in half crosswise. Cut fish into eight strips. Place a fish strip on the entire length of the noodle, fold the fish, as needed. Place 1/8th of the spinach at the beginning end of each lasagna. Roll up and place the rolls, seam side down, in a 38 x 25-cm (15 X 10-inch) baking dish, sprayed with oil. Cover with the sauce and cheese.

Bake for about 25 minutes and finish cooking under the broiler to brown the cheese.