Zest the lemon into a small bowl, then squeeze at least 1/4 cup juice into a separate bowl. Set zest aside.
Whisk together lemon juice, 1/4 cup olive oil, minced garlic (2 cloves), oregano, salt, pepper, and a pinch of lemon zest to make the dressing.
Mince 1 more garlic clove. Combine half the dressing with the chicken thighs and minced garlic in a zip-top bag. Marinate for at least 30 minutes.
Cook pasta according to package directions. Drain and let cool completely.
Heat a skillet over medium heat. Cook marinated chicken thighs for about 5 minutes per side until cooked through. Let rest, then chop into bite-sized pieces.
Dice cucumber and bell pepper. If using tomatoes, halve or dice them. Chop parsley. Prepare any other chosen veggies.
In a large bowl, combine cooled pasta, chopped cucumber, bell pepper, tomatoes, olives, parsley, and chicken. Pour remaining dressing over the salad, add lemon zest, and toss to combine.