Preheat the oven to 400 degrees F.
In a large bowl, stir the cumin, paprika, sumac,1 teaspoon kosher salt, 1/2 teaspoon black pepper and 2 tablespoons olive oil.
Add the sweet potatoes, carrots and cauliflower and toss them in the spice-oil mixture. Spread them across 2 sheet pans in a single layer.
Roast the vegetables until they are tender in the middle and lightly browned at the edges, flipping over and tossing halfway through roasting, about 30-35 minutes.
For the couscous, combine the couscous grains and boiling water in a medium bowl. Cover and let sit for 5 minutes. Use a fork to comb through the couscous, breaking up any clumps.
To make the vinaigrette, whisk the garlic, mustard, salt, pepper, lemon juice and olive oil in small bowl.
In a large bowl combine the couscous, roasted vegetables, scallions, parsley, pistachios and feta. Drizzle in the dressing, tossing to combine.