In a small bowl, whisk together Dijon, garlic, marjoram, basil, oregano, vinegar, salt and pepper. Slowly drizzle in olive oil while whisking to form an emulsion. Set dressing aside, allowing flavors to blend.
Place a large skillet over medium heat then add olive oil. When oil is hot, add the bread cubes and toss to coat in the oil. Sprinkle bread lightly with salt and pepper and cook, tossing frequently, for about 10 minutes, until most of the bread is lightly browned and crispy. Remove from heat then set aside to cool.
Cut Romaine, cucumber, bell pepper, tomatoes and grilled chicken into ½" chunks, then place in a large salad bowl. Pour vinaigrette over ingredients in the bowl then toss to combine.
Add cooled bread cubes, feta and Kalamata olives and toss lightly. Cover and refrigerate for 15–20 minutes to let the flavors blend and bread to absorb the dressing. Toss once or twice before serving to ensure bread absorbs dressing evenly.
Notes
Get creative and add any of your favorite ingredients for a Greek salad like onions or beans!