Bring a large pot of salted water to a boil over high heat.
Turn off the heat, add 1 pound peeled and deveined raw medium shrimp, and poach until the shrimp are opaque and just cooked through, 2 to 3 minutes.
Drain the shrimp and set aside until cool enough to handle, about 10 minutes.
Chop the shrimp into ½-inch pieces and place in a large bowl.
Add ¼ cup freshly squeezed lemon juice, ¼ cup freshly squeezed lime juice, 2 chopped tomatoes, finely chopped red onion, 1 finely chopped medium jalapeño, ½ cup chopped cilantro, 1 diced cucumber, 2 diced mangos, and ½ teaspoon kosher salt, and toss to combine.
Cover and refrigerate for at least 1 hour or up to 4 hours.
Just before serving, dice 1 medium avocado, add to the ceviche, and gently toss to combine.
Serve with tostadas or tortilla chips, if desired.
