Heat oven to 220C/400F. Toss the eggplant cubes with cooking spray, chopped onion, and whole garlic cloves in a roasting pan and season with salt and pepper. Roast for 15 minutes.
Remove the garlic and place it on a separate plate. Using a fork, mash it and then coat the eggplant mixture with the mashed garlic. Set aside to cool.
Put the couscous and water and a pinch of salt into a bowl and cover for 5 minutes. Fluff with a fork and transfer to a salad bowl.
Combine all of the dressing ingredients in a small bowl.
Add the eggplant mixture, feta, herbs and olives to the couscous. gently mix to combine. Add the roasted red peppers and dressing and toss to coat. Taste for seasoning and add more if necessary.