Combine the couscous and 3 cups boiling water in a heatproof bowl. Cover and let stand until the water is absorbed, about 15 minutes. Fluff with a fork, then stir in the butter, turmeric and salt. Cover and set aside.
For the vegetable stew, heat the oil in a large saucepan or soup pot. Add the onions and sauté over medium heat until translucent. Stir in cabbage and sauté until cabbage and onion are lightly golden.
Add the remaining stew ingredients. Bring to a simmer, then cover and reduce the heat to medium-low. Cook, stirring occasionally, for 15 to 20 minutes. Add water as needed to produce a moist, but not soupy, consistency. The vegetables should be just tender, but still firm.
To serve, arrange the couscous on the outer edge of a large serving platter and make a well in the center. Pour the vegetable mixture in the center, then sprinkle with the garnishes.