Combine tomatoes, 1/2 cup diced red onion, pineapple, and cilantro in a medium bowl. Add 1 tablespoon lime juice, salt, and sugar; toss to combine. Cover salsa with plastic wrap and place in the fridge for at least 2 hours.

Place the remaining red onion in the bowl of a small food processor. Add juice of one lime and process until finely ground, about 3 minutes.

Place chicken strips into a shallow dish. Slather onion mixture over the chicken, and refrigerate for at least 2 hours.

Preheat an outdoor grill to high heat, about 400 degrees F (200 degrees C). Lightly oil the grill.

Cook until chicken is no longer pink in the center and the juices run clear, 3 to 4 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Remove to a plate.

Lightly brown the tortillas on the grill for 30 seconds to 1 minute. Drain the salsa.

Thinly slice the chicken, against the grain. Lay tortillas out on plates, top with lettuce, chicken, pineapple salsa, and Sriracha sauce.