Always kick off tarhana soup with cold water, whisking steadily right as it hits the pot. If you pour in hot liquid, those dried fermented grains will seize up into stubborn little clumps before having a chance to dissolve. Keep your whisk moving until the pot hits a full boil, then dial the flame down and let it gently simmer away for five minutes.
Letting the dried mint and tomato paste sizzle briefly in warm oil before introducing the liquid makes a world of difference for flavor. Just keep a close eye on the mint - you want that rich, blooming aroma, not scorched herbs. Don't worry if things look watery early on; tarhana works its thickening magic as it simmers and will keep bodying up as it sits. A splash of water when warming up leftovers will loosen it right back up.