Heat oil in large skillet over medium heat. Add onions, sautéing until softened. Add sausage, cooking until browned. Add pasta sauce, stirring to combine. Reduce heat to a simmer.
Boil broken lasagna noodles for 5 minutes to soften slightly. Drain and set aside.
In a large mixing bowl, combine ricotta cheese, parmesan, salt and pepper.
Spoon about 1 cup of meat sauce into the bottom of a 12 inch cast iron skillet. Layer evenly with broken pieces of semi-cooked lasagna noodles. Spoon about 8 mounds of ricotta cheese over noodle layer then sprinkle with additional Parmesan and shredded mozzarella cheese. Top with 1 1/2 cups of meat sauce. Continue this layering until you reach the top of the skillet. Top layer should be a layer of noodles, sauce then shredded mozzarella cheese.
Bake skillet lasagna for 20-30 minutes, until cheese is melted and sauce is bubbly around edges. Remove and let cool 10 minutes.