Preheat oven to 375 degrees. Place the squash, onion, carrots and zucchini on a parchment-lined jelly roll pan. Add the olive oil, salt and pepper; toss. Roast the vegetables for about 25-30 minutes turning once at the halfway point for even roasting.
While the vegetables roast, bring the vegetable broth to a boil in a saucepan. Remove pan from the heat and stir in the butter/butter substitute, some more cracked black pepper, ground cumin, saffron threads; add salt, to taste. Cover the pan and allow the mixture to steep for 15 minutes.
Transfer the roasted vegetables and their juices to a large bowl. Stir in the couscous. Bring the broth back to a boil then pour it over the couscous and roasted vegetables.
Cover the bowl tightly with a plate or lid and let stand for 15 minutes. Add the scallions, stir gently and serve immediately.