Scatter the carrots and cauliflower in an even layer on a baking sheet. Drizzle with 3 tablespoons oil and sprinkle with 1 teaspoon salt and a few cracks of pepper. Bake, stirring halfway through, until charred and tender, about 1 hour 15 minutes.
Meanwhile, bring a medium saucepan full of water to a boil. Salt the water generously. Add the couscous and boil until it is cooked to al dente, 8 to 10 minutes. Drain the couscous and transfer to a serving bowl. Toss with the raisins, half the lemon zest and juice, 1 tablespoon olive oil, 1 tablespoon torn mint, 1 tablespoon torn parsley leaves and a pinch of salt and pepper.
Add the yogurt, remaining lemon zest and juice, 2 tablespoons oil and 1 tablespoon water to a small bowl and whisk to combine. Add a pinch of salt and pepper and more water if you want a thinner sauce.
Serve the vegetables with the couscous and sauce. Sprinkle with the remaining herbs 2 tablespoons mint leaves and 1 tablespoon parsley.