**Prepare the Vegetables:**
Wash and chop all the vegetables. Combine romaine lettuce (if using), cherry tomatoes, cucumber, red onion, green bell pepper, and olives in a large salad bowl.
**Add the Cheese:**
Top the salad with cubed or crumbled feta cheese.
**Make the Dressing:**
In a small mixing bowl or jar, whisk together olive oil, red wine vinegar, minced garlic, oregano, Dijon mustard (if using), salt, and pepper until well combined.
**Assemble the Salad:**
Drizzle the dressing over the salad just before serving. Toss gently to ensure the dressing coats all the ingredients evenly.
**Serve:**
Serve immediately as a side dish or light main course.