Position oven rack in center of oven; preheat oven to broil.

Bring a large saucepan of water to a boil over high heat. Cook 2 (9-ounce) packages ravioli according to package directions; drain well.

Meanwhile, heat 1 tablespoon oil in a medium, high-sided ovenproof skillet over medium-high heat until shimmering.

Add 2½ cups mushrooms in an even layer; cook, undisturbed, until browned on the bottom, about 5 minutes.

Stir in 1 cup onion; cook, stirring occasionally, until the mushrooms are browned and the onions are softened, about 4 minutes.

Gradually stir in 5 cups baby spinach, 1 tablespoon garlic and ¼ teaspoon crushed red pepper; cook, stirring constantly, until the spinach is wilted, about 2 minutes. Transfer to a medium bowl. Wipe the skillet clean.

To the same skillet, add 1 (28-ounce) can crushed tomatoes, ¼ cup basil and ⅛ teaspoon salt; bring to a simmer over medium-high heat, stirring occasionally.

Remove from heat; stir in the cooked ravioli. Transfer to a separate medium bowl.

Spoon half of the ravioli mixture (about 3 cups) in an even layer in the bottom of the skillet.

Spoon half of the spinach mixture (about 1 cup) in an even layer over the ravioli mixture.

Repeat the layers once more with remaining ravioli mixture and spinach mixture.

Arrange ½ cup torn mozzarella in an even layer over the spinach mixture.

Broil until the cheese is melted and lightly browned, 3 to 5 minutes.

Let stand for 5 minutes before serving. Sprinkle with the remaining ⅛ teaspoon salt. Garnish with fresh basil leaves, if desired.