Preheat grill to medium-high heat and lightly oil the grates.
Brush chicken breasts with olive oil and season with salt, pepper, and garlic powder.
Grill chicken for 5–6 minutes per side, or until fully cooked. Remove and rest for 5 minutes before slicing.
In a large bowl, add salad greens, cherry tomatoes, red onion, and avocado.
In a small bowl, whisk together dressing ingredients until emulsified.
Slice grilled chicken against the grain and add to the salad.
Drizzle dressing over salad and toss gently just before serving.