Season the lamb with 1 teaspoon salt and several grindings of pepper. In a large skillet or sauté pan, heat 1 tablespoon of the oil over high heat. When it shimmers, add the lamb and cook, turning with a thin metal spatula, until the pieces are browned on at least two sides, about 5 minutes total. Transfer to the slow cooker.
Return the skillet to medium heat and add 1 tablespoon oil, the onion, and a pinch of salt. Cook, stirring, for 3 minutes, add the turmeric and stir for 30 seconds, then pour in ½ cup (120 ml) water and scrape up any browned bits. Scrape the onion mixture into the cooker.
Finely chop the thickest parsley stems from the bunches and add them to the cooker. Slice half of the rhubarb cross- wise ½ inch (12 mm) thick and add the slices to the cooker. Cover and cook on low for 8 hours.
Morning or Evening
Finely chop the remaining parsley and the mint leaves together. Slice the remaining rhubarb on the bias ¼ inch (6 mm) thick. If doing this in the morning, put the herbs and rhubarb in containers and refrigerate.
In a skillet or sauté pan, heat the remaining 1 tablespoon oil over medium-high heat. When it shimmers, add the rhubarb and a pinch of salt and cook, stirring frequently, until just tender, about 5 minutes, adding the sugar in the last minute. Gently fold the rhubarb and the parsley and mint into the stew. Season with more salt and sugar, if needed, then serve.