2. Turn the oven temperature down to 350 degrees.
3. In a large skillet over medium heat, heat 2 tablespoons of the olive oil. Add the onions, salt, and pepper. Cook, stirring often, for 5 minutes. Add the mushrooms and continue cooking, stirring often, for 5 minutes or until they release their juices. Turn up the heat and cook, stirring, until the juices evaporate.
4. Remove the mixture from the skillet. Wipe it out. Add 2 tablespoons of the oil. Cook the spinach, stirring constantly, for 2 minutes or until it wilts. Add salt and pepper.
5. With tongs, transfer the spinach to a board, leaving any liquid in the pan. Let the spinach cool. Chop it coarsely. Transfer to a bowl. Add the ricotta, eggs, Parmesan, salt, and pepper.
6. Wipe out the skillet. Add the butter and when it melts, whisk in the flour. Cook, whisking constantly, for 2 minutes. Gradually whisk in the milk. When it comes to a boil, add salt and pepper, lower the heat, and simmer, whisking often, for 2 minutes.
7. Spread enough white sauce in the baking dish to make a thin layer. Add enough sheets of lasagna to cover the bottom, breaking them if necessary to fit. Dot with 1/3 of the sauce, 1/2 of the ricotta, 1/2 of the mushroom mixture, 1/2 of the roasted asparagus, and 1/3 of the mozzarella.
8. Add another layer of lasagna to cover the filling, 1/3 of the sauce, the remaining ricotta, remaining mushroom mixture, remaining roasted asparagus, and 1/3 of the mozzarella.
9. Add another layer of lasagna to cover the filling, spread with remaining sauce, and sprinkle with remaining mozzarella. Toss asparagus tips with remaining 1 tablespoon olive oil. Set on top.
10. Cover with foil. Set the dish on a rimmed baking sheet. Bake for 70 minutes, or until the pasta is tender when pierced with a skewer.
11. Turn the oven temperature to 425 degrees. Uncover the lasagna and continue baking for 10 minutes or until the top is brown.
Adapted from Verrill Farm
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