To prepare vinaigrette, mix mustard, vinegar, lemon juice, garlic powder, onion powder, and oregano in a bowl. Slowly add olive oil, whisking vigorously to ensure vinaigrette is well-combined. Season to taste with salt and pepper.
To serve, toss cucumber, chickpeas, bell peppers, cherry tomatoes, red onions, olives, feta and parsley with vinaigrette.
