Preheat the Oven and Cook Ravioli
Position the oven rack in the center and preheat the oven to broil.
Bring a large saucepan of water to a boil. Cook the ravioli according to the package directions until tender. Drain well and set aside.
Cook the Mushroom and Spinach Mixture
Heat the olive oil in a medium, high-sided ovenproof skillet over medium-high heat.
Spread the mushrooms in an even layer and cook undisturbed for about 5 minutes until browned on the bottom.
Add the chopped onion and cook, stirring occasionally, until the mushrooms are browned, and the onions are softened, about 4 minutes.
Stir in the spinach, garlic, and crushed red pepper, cooking constantly until the spinach is wilted, about 2 minutes.
Transfer the mixture to a medium bowl and wipe the skillet clean.
Prepare the Tomato Sauce
In the same skillet, combine the crushed tomatoes, chopped basil, and ⅛ teaspoon salt.
Bring the mixture to a simmer over medium-high heat, stirring occasionally.
Remove the skillet from heat and stir in the cooked ravioli.
Transfer the ravioli mixture to a separate medium bowl.
Assemble the Lasagna
Spread half of the ravioli mixture (about 3 cups) evenly in the bottom of the skillet.
Layer half of the spinach mixture (about 1 cup) over the ravioli.
Repeat the layers with the remaining ravioli mixture and spinach mixture.
Tear the mozzarella slices and arrange them evenly over the top.
Broil the Lasagna
Place the skillet under the broiler for 3 to 5 minutes, or until the cheese is melted and lightly browned.
Serve and Garnish
Let the lasagna stand for 5 minutes before serving. Sprinkle with the remaining ⅛ teaspoon salt and garnish with fresh basil leaves, if desired.
