Bring a large pot of salted water to a boil over high heat. Turn off the heat, add the shrimp, and poach until opaque and just cooked through, about 2 to 3 minutes.

Drain 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 the lemon juice, lime juice, tomatoes, red onion, jalapeño, cilantro, and kosher salt to the shrimp, and toss to combine.

Cover and refrigerate for at least 1 hour or up to 4 hours.

Just before serving, dice the avocado, add it to the ceviche, and gently toss to combine.

Serve with tostadas or tortilla chips, if desired.