Place bell peppers and tomatoes in glass or plastic dish. In a tightly covered container, shake vinegar, oil, oregano, salt, pepper, sugar, mustard, and garlic. Pour vinegar mixture over vegetables. Cover and refrigerate for 1 hour or more to blend flavors.
Line a serving platter with spinach. Drain vegetables and place on top of spinach. Sprinkle with cheese. Garnish with olives (optional).