Add the cucumber, red bell pepper, yellow bell pepper, cherry tomatoes, and red onion to a large mixing bowl.
In a small bowl, combine the minced garlic, dried oregano, Dijon mustard, red wine vinegar, salt, ground black pepper, and olive oil. Whisk well.
Pour the vinaigrette over the vegetables and toss well with a spoon and fork. Top with feta cheese. Let the salad sit for 10 minutes to allow the flavors to blend, then serve.