1. Peel and dice all vegetables. Slice chicken into thin strips.
2. Pour broth in a saucepan. Add bay leaf and all vegetables except courgette. Cook for 10 minutes over low heat, then add courgette and chicken breast. Cook for 5 more minutes. Check that the vegetables are well cooked.
3. Strain vegetables from broth and set aside them aside.
4. Return broth to saucepan and dissolve flour in broth. Add egg white and stir. Cook for 1 minute until thickens slightly.