Bring a large pot of well-salted water to a boil. Meanwhile, prepare an ice bath by filling a large bowl with ice water. Add the shrimp all at once to the boiling water and cook until the shrimp are just pink and curled into a C shape, 1 to 2 minutes. Immediately drain the shrimp and place in the ice bath to cool. Do not overcook!
After the shrimp have rested a few minutes in the ice bath, drain them, remove the tails if necessary, and chop into 1/4-inch pieces. Pat dry.
In a large bowl, combine the shrimp, avocado, tomatoes, green onions, jalapeno (if using), lime juice, cilantro, and 3/4 teaspoon salt. Stir well, cover, and refrigerate for at least 20 minutes, until chilled and the flavors are married. Taste and add additional salt if desired. Serve with tortilla chips.
Notes
*To prepare with raw shrimp: Juice 1 orange into a bowl. Add the shrimp and lime juice. Marinate in a covered container in the fridge for 1 to 2 hours, until the shrimp are pink and curled into a C shape. Proceed by adding the remaining ingredients.
*You can start with pre-cooked shrimp and skip the poaching in step 1.
TO STORE: If you make your ceviche with the cooked shrimp, it will last for 2 days in an airtight container in the refrigerator. If you make it with raw shrimp, it will last 1 day.