Homemade vegetable stock is best in this recipe. If using store-bought stock, upgrade the flavor by adding a bay leaf while the broth simmers in step 2.
Heat olive oil in a large saucepan over medium. Add onions; cover and cook, stirring occasionally, until soft and translucent, about 15 minutes.
Stir in stock, garlic, turmeric, ginger, salt, and saffron. Bring to a boil over high. Cover and reduce heat to low; simmer gently 30 minutes.
Add turnips, and bring mixture to a boil over high. Cover, reduce heat to medium-low, and cook 10 minutes. Add potatoes, celery root, and parsnips, and stir to combine. (There should be enough liquid in pan to almost cover the vegetables; if not, pour in just enough water to do so.) Bring to a boil over high. Cover and reduce heat to medium-low; cook until vegetables are cooked through and softened, about 20 to 25 minutes.
Meanwhile, place dried plums in a heatproof bowl, and add boiling water to cover. Let stand until plums are softened, about 5 minutes; drain.
Remove saucepan from heat, and use a ladle to transfer 3/4 cup broth to a small saucepan over medium. Add drained plums, honey, and cinnamon to broth. Simmer mixture, stirring occasionally, until sauce has thickened and reduced by half, about 15 minutes.
Divide couscous evenly among 6 shallow bowls. Top with vegetables, plum mixture, and cilantro leaves.