Bring a large pot of salted water to a boil. Cook the pasta according to package directions, cooking them until past the al dente stage, and in fact, nicely tender, so that far end of the recommended cooking time, plus an extra minute. Drain, and rinse quickly with cool water.
While the pasta is cooking, in a large bowl combine the olive oil, lime juice, scallions, bell pepper, cilantro (if using) and salt and pepper. Once you have drained and rinsed the pasta, add it to the bowl with the dressing and toss to coat well. Allow it to sit for about 5 minutes.
Add the chicken, avocado and tomatoes and toss to combine everything. Taste and adjust the seasonings as needed. Transfer to a serving bowl.
Notes
If you want to make this ahead of time, it’s perfect for that. You can make it all a day ahead of time, and just add the avocado and the tomato at the end. Keep it, covered in the fridge. The tomatoes can also be included ahead of time if it makes your life easier, but I never like to put fresh tomatoes in the fridge if I can help it, whether they are uncut, or already part of a dish. Their texture gets affected, and even if you bring them back to room temp before serving, I think their flavor gets muted as well.