In a 12-inch cast-iron skillet, heat oil over medium heat. Add beef; cook, stirring occasionally, until beef is browned and crumbly, about 12 minutes. Remove beef using a slotted spoon, and let drain on paper towels, reserving drippings in skillet.
Add squash, mushrooms, onion, garlic, salt, 1 teaspoon Italian seasoning, and peppers to skillet. Cook over medium heat, stirring occasionally, until vegetables are just tender, about 5 minutes. Stir in beef, marinara sauce, and spinach; cook, stirring occasionally, until heated through, 5 to 6 minutes.
Reserve 5 cups meat sauce from skillet in a medium bowl. Spread half of tortellini on meat sauce in skillet. Spread 1½ cups meat sauce onto pasta.
In a medium bowl, whisk together ricotta, Parmesan, egg, and remaining ¼ teaspoon Italian seasoning. Spread ricotta mixture onto sauce in skillet. Sprinkle with half of mozzarella. Spread 1½ cups meat sauce onto mozzarella. Top with remaining tortellini and remaining meat sauce. Loosely cover skillet with foil, and place on a large baking sheet. Bake until hot and bubbly, about 1 hour. Uncover and sprinkle with remaining mozzarella. Bake until cheese is melted, about 15 minutes more. Let stand 15 minutes before serving. Garnish with basil and Parmesan, if desired.
Notes
It’s not necessary to boil the tortellini before assembling the lasagna; the pasta will cook as the casserole bakes. Refrigerated ravioli can also be used.
Recipe by Southern Cast Iron at https://southerncastiron.com/tortellini-vegetable-lasagna/