Preheat the oven to 150°C (300°F/Gas 2). Line a baking tray.
Chop the eggplant into small cubes and place in a large mixing bowl with 1 teaspoon of the ground coriander and a little salt. Pour 2 tablespoons of the oil onto the eggplant, mix, then sprinkle on another teaspoon of coriander and another tablespoon of oil.
Mix until the cubes are roughly coated with oil and spice and then place on the tray and into the oven. Roast for 25 minutes, then turn the eggplant cubes over and return to the oven again for another 25 minutes, or until brown, taking care to avoid burning. Set aside.
While the eggplant is roasting, gently sauté the onion in a saucepan with the remaining oil until soft and golden, then add the remaining ground coriander, the black pepper and garlic. Stir for 1 minute, or until fragrant.
Add the couscous and stir until the grains are coated with the spice mix and lightly toasted. Add enough stock to cover the couscous and bring to the boil. Reduce the heat to very low and cover with a lid.
After 10 minutes, check the couscous, using a fork to fluff the grains. Loosen any grains stuck to the bottom of the pan. Cover again and cook for another 5 minutes. Fluff again and set aside, covered.
In a large bowl, mix together the couscous, herbs and eggplant. Stir in the lemon juice and a little extra olive oil if needed and season to taste.