Step 1: Combine the rhubarb and water in a heavy nonreactive saucepan and cook, uncovered, over medium heat until the rhubarb is soft and mushy, 8 minutes.
Step 2: Stir in the lemon juice, oil, garlic, coriander, salt, and pepper flakes and simmer for 3 minutes. Transfer the mixture to a food processor and process to a smooth puree. Return the puree to the saucepan and stir in the cilantro and dill.
Step 3: Continue simmering the sauce until thick and flavorful, 3 minutes. Correct the seasoning, adding salt or sugar to taste; tkemali should be highly seasoned and tart (but not unpalatably sour). Let cool to room temperature and serve. Refrigerated, the sauce will keep for at least 1 week.
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Steven Raichlen’s Project Smoke is a production of Maryland Public Television, Barbacoa, Inc., and Resolution Pictures. © 2017 Barbacoa, Inc.