1. Bring a large pot of water to a boil. Separate cabbage into leaves. Add leaves to water in batches; boil 5 minutes. Remove and rinse under cold water. Reserve the water in the pot.
2. In a large skillet, heat oil over medium heat. Add onion; sauté 5 minutes. Add garlic, spinach and parsley; cook, stirring, 4 minutes. Stir in bread, vinegar, and 2 tablespoons of reserved water. In a blender or food processor, combine bread mixture and egg; process until smooth.
3. Line a large bowl with enough cheesecloth to leave a 6-inch overhang. Reconstruct cabbage in bowl, starting with the largest leaves on outside and ending with the smaller leaves. Shape bread stuffing into a ball; place in center of cabbage. Bring leaves up to enclose stuffing and bring cheesecloth up around cabbage; tie securely.
4. Return cabbage to reserved cooking water; simmer 15 minutes. Remove from pot; cool slightly. Unwrap and cut into wedges.