Warm the broth and water in a saucepan until gently simmering. Meanwhile, beat the eggs in a small bowl until the white and yolk are well combined.
Stir the broth slowly in one direction. Pour in the beaten eggs in a thin stream, then stop stirring for a few seconds so the ribbons can form.
Keep at a gentle simmer for another 1–2 minutes, until the egg is fully set. Taste, add pepper and parsley if using, and divide between two bowls. Serve straight away.