Preheat grill or grill pan to medium-high heat. In medium bowl, sprinkle chicken with oregano, coriander and salt and pepper, to taste. Add oil then toss to coat.
Grill chicken, covered, about 8 minutes per side, until slightly charred and registering 165 F on thermometer inserted into thickest part. Transfer to cutting board, let rest 5 minutes then thinly slice.
Divide romaine, spinach, avocados, cucumbers, apples and grapes among bowls. Top with chicken.
To make dressing: Whisk Greek yogurt, olive oil, honey, Dijon mustard, lemon juice, apple cider vinegar, garlic, salt and pepper. Drizzle over salad and serve.