Prepare ravioli according to package directions. Drain and set aside.

In a large skillet, heat oil and add onion. Cook until onion is tender.

Add squash. Sauté, stirring occasionally, about 5 minutes until squash becomes tender.

Add salt and pepper.

In a small bowl, combine milk, flour, and butter; mix until well blended. Pour milk mixture and lemon juice into squash pan and cook over medium heat about 3-5 minutes until slightly thickened.

In a large bowl or in the pan the ravioli was cooked in, mix together the cooked, drained ravioli with the squash mixture. Add grated parmesan cheese and stir until cheese has melted.

Serve warm with additional cheese if desired.