Preheat the oven to 425 degrees F. Toss the carrots and onion on a rimmed baking sheet with 1 tablespoon olive oil, 1/2 teaspoon salt, and pepper to taste. Roast until tender and slightly golden, about 25 minutes. Meanwhile, cook the couscous as the label directs, adding the raisins in the last 5 minutes of cooking. Drain the couscous and raisins and rinse under cold water. Make the dressing: In a small bowl, whisk the yogurt with the remaining 2 tablespoons olive oil, the harissa, vinegar, 1/2 teaspoon salt and 2 to 3 tablespoons water until smooth. Put the escarole in a large bowl and add the roasted carrots and onion; toss to wilt the greens slightly. Add the couscous-raisin mixture, the dressing and the roast beef and toss to coat. Divide the salad among plates.