Combine in a bowl the onion, tomato, and serrano pepper.
Add lime juice, tuna, ketchup and cilantro. Set aside.
Defrost sweet peas and corn following microwave directions from their package; let them cool down. Add celery, green peppers, sweet peas, and corn, to onion-tomato mixture. Salt to taste.
Mix well, chill for 15-30 minutes.
Just before serving add lettuce and avocado. Serve on tostadas or with saltine crackers.