Cook pasta as per instructions. Drain, put back in large pot.
In a skillet, heat the oil, add the onions, cook for 3-4 minutes.
Add the frozen peas and shrimp, cook until thawed and cooked through (if raw). Remove from heat.
Add the pesto and lemon juice to the pot with the pasta, mix well. Add the skillet mixture to the pot. Mix gently.
Serve warm, top with grated parmesan cheese.