Step 1) Heat oil in a stock pot or large pan. Add onion and garlic; cook for 3 minutes until clear. Add carrots, celery and chayote. Cook for 5 minutes while stirring occasionally.
Step 2) Add shrimp, bay leaf, water, bouillon and Serrano pepper, if desired. Cover and simmer for 10 minutes.
*Note: Unpeeled shrimp are more work to eat, but the shells impart more flavor to the finished dish.