Heat oil in a 12 inch cast iron skillet over medium heat and sauté onions for 4 minutes. Add butternut squash cubes, garlic, two tablespoons of water and 1 teaspoon of salt. Cover and cook for 10-12 minutes, stirring occasionally, until fork tender. Add the spinach, stir and cook an additional minute uncovered.
Remove squash and spinach mixture to a medium bowl and cool.
In a separate bowl stir the ricotta, parmesan, Italian herbs, cooled squash mixture and 1/2 teaspoon salt and pepper to taste.
Place 1/2 cup of the marinara in the same cast-iron skillet and top with 2 pasta noodles. Spread 1/2 of the ricotta mixture on top and sprinkle with 1/2 of the mozzarella cheese.
Top with 1 cup marinara, 2 more noodles, 1/2 of ricotta mixture, remaining noodles, marinara and remaining mozzarella.
Place pan over medium low heat, cover and cook for 25-30 minutes or until pasta is fork tender.