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 1/2-inch pieces and place in a large bowl. Add 1/4 cup freshly squeezed lemon juice, 1/4 cup freshly squeezed lime juice, 2 chopped tomatoes, finely chopped red onion, 1 finely chopped medium jalapeño, 1/2 cup chopped cilantro, and 1/2 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.