1. Lightly coat a 9-by-13-inch baking dish with nonstick cooking spray or oil. In a large saucepan, warm the olive oil over medium heat. Add the onion and sauté for 3 to 5 minutes, or until softened. Add the garlic and sauté for 1 minute more, making sure it does not brown. Add the salt and stock or broth and bring to a rolling boil over medium heat.

2. In a thin stream (a measuring cup with a lip works well for pouring), very slowly add the polenta, stirring constantly with a wooden spoon. Reduce the heat to low and continue cooking for 3 to 5 minutes, stirring constantly to be sure it doesn't stick, until it is very thick, smooth, and creamy. Stir in 3 tablespoons of the Parmesan cheese.

3. Pour the polenta into the prepared baking dish, smoothing the top with a rubber spatula if necessary. Let the polenta rest for at least 2 hours to set.

4. Squeeze out all of the water from the spinach. In a small bowl, mix the spinach with the ricotta cheese. Season with salt, pepper, and nutmeg.

5. Preheat the oven to 375°E Invert the polenta onto a cutting surface. Set the dish aside to use again. Cut the polenta rectangle in half crosswise to make 2 rectangles each measuring 9 by 6 1/2 inches. (This will make it easier to transfer the polenta back into the baking dish later on.) Cut a 2-foot long piece of dental floss and hold it taut between your hands. Working with 1 piece of polenta at a time, place the floss against the end farthest from you and pull the floss toward you, through the center of the polenta piece, slicing it in half to give you a top and a bottom layer. (Alternatively, use a serrated knife to slice through the polenta.) Repeat with the second rectangle.

6. Spoon 1 cup of the Red Pepper-Tomato Sauce evenly on the bottom of the same baking dish. Put 2 of the polenta pieces in the dish to cover the bottom. Spoon half of the spinach-ricotta mixture over the polenta, using the back of a spoon to spread it out, then sprinkle with 1 cup of the mozzarella and 2 tablespoons of the Parmesan cheese. Spoon 2 cups of the sauce over the cheese.

7. Top with the remaining polenta pieces, covering the sauce completely. Spoon the remaining spinach-ricotta mixture evenly over the polenta. Sprinkle the remaining 1 cup mozzarella over the top. Dot with the remaining 1 cup sauce, leaving small gaps between the sauce. Sprinkle with the remaining 2 tablespoons Parmesan. (The lasagna will be high, slightly above the rim of the dish.)

8. Put the lasagna dish on a baking sheet to catch any drips. Bake, uncovered, for 30 minutes. Remove from the oven and let sit for 10 minutes. To serve, cut into serving portions and use a spoon or spatula to scoop them out onto individual dishes.

To make ahead: Prepare through Step 7, cover, and refrigerate for up to 1 day. Remove from the refrigerator 1 hour before baking and proceed with Step 8. Freeze in an airtight container for up to 1 month.
To reheat: Bake in a preheated 350°F oven until heated through, 15 to 20 minutes.