Combine the couscous and 3 cups boiling water in a heatproof bowl. Cover and let stand until water is absorbed, about 10 minutes. Fluff with a fork, then stir in the margarine, 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 moderate heat until translucent. Stir in cabbage and sauté until both it and the onion are lightly golden.
Add remaining stew ingredients. Bring to a simmer, then over 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 tender, but still firm.
To serve, arrange the couscous on the outer edge of a large serving platter. Pour vegetable mixture in the center, then sprinkle with the garnishes. Let each guest place a mound of couscous on their dinner plate and top it with the vegetable mixture.