Place the chicken breasts in a bowl and drizzle with olive oil. Add the Greek seasoning, salt, and pepper, then toss until evenly coated. Let sit for at least 10–15 minutes while you prep the rest.
Heat a skillet or grill over medium heat. Add the chicken and cook for about 6–7 minutes per side, depending on thickness, until the internal temperature reaches 165°F. Remove from heat and let rest for 5–10 minutes, then cut into bite-sized pieces.
Meanwhile, bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions until al dente. Drain and rinse lightly with cool water to stop the cooking, then set aside.
While the pasta cooks, dice the cucumber, halve the cherry tomatoes, finely dice the red onion, and chop the fresh dill.
In a small bowl or jar, whisk together the olive oil, lemon juice, Dijon mustard, water, and Greek seasoning until smooth and well combined.
In a large bowl, add the cooked pasta, cucumber, tomatoes, red onion, olives, feta, dill, and grilled chicken.
Pour the dressing over the salad and toss until everything is evenly coated.
Taste and adjust seasoning with additional salt, pepper, lemon juice, or feta as needed.
Serve immediately, or refrigerate for 30–60 minutes to allow the flavors to come together before serving.