Preheat the oven to 375°F (190°C). Season the chicken with salt and pepper, and roast in a pan for about 45 minutes or until fully cooked. Let it cool before shredding.

In a mixing bowl, whisk together the Greek yogurt, olive oil, lemon juice, minced garlic, Dijon mustard, salt, and pepper until smooth.

In a large salad bowl, combine the mixed greens, cherry tomatoes, cucumber, avocado, red onion, and shredded chicken. Drizzle with the yogurt dressing and toss gently to combine.