Soak the sun-dried tomatoes in the boiling water until soft. In a mortar and pestle, pound the sun-dried tomatoes with their liquid to a paste.
In a heavy-bottomed soup pot, saute onion and leek in olive oil until translucent.
Add diced tomatoes. Add sun-dried tomato paste and sugar. Continue cooking over a moderately high heat for 2-3 minutes. Stir in the garlic, grated orange zest, and mint, and cook another 2-3 minutes. Gradually add the herb broth, cover, and simmer for 30-45 minutes longer. (Soup may be prepared ahead of time up to this point.)
In a bowl, mix together yogurt and flour until smooth. Gradually add a spoonful of hot soup, stirring contantly. Add another spoonful of soup, then slowly add yogurt mixture to soup pot, stirring constantly. Continue cooking soup over a gentle heat, stirring, being careful not to let mixture curdle.
Adjust seasonings, then add the fresh parsley. Serve immediately.