- In a pot over medium heat, soften the onion and garlic in the oil for 5 minutes. Add the tomatoes and oregano. Bring to a boil. Simmer for 15 minutes. Season with salt and pepper. Set aside.
- Meanwhile, in a large pot over medium-high heat, soften the spinach and garlic in the oil for 2 minutes or until the spinach has wilted. Transfer to a strainer and press out the liquid. Let cool. Coarsely chop.
- In a bowl, combine the spinach with the remaining ingredients. Season with salt and pepper. Set aside.
- With the rack in the middle position, preheat the oven to 375°F (190°C).
- Cover the bottom of a 13 x 9-inch (33 x 23 cm) baking dish with 1 cup (250 ml) of the tomato sauce. Cover with a layer of noodles. Cover with 1 ½ cups (375 ml) of the sauce, 1/4 cup (25 g) of the mozzarella and 2 tbsp of the Parmesan. Cover with another layer of noodles and half of the spinach filling. Repeat the last two steps. Finish off with the remaining noodles and tomato sauce. The lasagna can be frozen at this point. Top with the remaining mozzarella. Place the dish on a baking sheet.
- Bake for 35 minutes (see note). Finish off under the broiler, if desired. Let sit for 15 minutes before serving.
If you assemble and refrigerate the lasagna in advance, add 10 minutes to the cooking time.