Part-skim ricotta and mozzarella keep the fat count low in these cheesy roll-ups, while a little bold-flavored Parmesan goes a long way flavor-wise.
Heat the oil in a large saute pan over medium-high heat. Add the mushrooms and cook, stirring occasionally, until browned and all the liquid has evaporated, about 5 minutes. Season with 1/4 teaspoon of the salt. Stir in 1 1/2 cups of the tomato sauce and simmer for 2 minutes. In a medium bowl, combine the ricotta cheese, spinach, egg, the remaining 1/4 teaspoon salt, a few turns of pepper and nutmeg.
Spread 1 cup of the tomato sauce on the bottom of a 9- by 12-inch baking dish. Spread about 2 tablespoons of the ricotta mixture onto a lasagna noodle. Top with about 1 1/2 tablespoons of the mushroom mixture, roll the noodle and place it into the baking dish. Repeat with the remaining noodles. Spread the remaining 1 1/2 cups sauce over the lasagna rolls. Top with the grated cheeses, cover loosely with foil and bake for 45 minutes. Uncover and bake for 15 minutes more.