Step 1

Combine in a bowl the onion, tomato, and serrano pepper.

Step 2

Add lime juice, tuna, ketchup and cilantro. Set aside.

Step 3

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.

Step 4

Mix well, chill for 15-30 minutes.

Step 5

Just before serving add lettuce and avocado.  Serve on tostadas or with saltine crackers.