Cut the aubergine and courgettes into 5 mm (¼ inch) thick slices and put in a large bowl with the red peppers. Add the olive oil and salt and pepper and stir well.
Heat a ridged griddle pan until hot. Add the vegetables, in batches, and cook for 3–4 minutes on each side, depending on size, until charred and tender.
Meanwhile, prepare the couscous. Put the couscous in a heatproof bowl. Pour over the boiling stock, cover and leave to soak for 5 minutes. Fluff up the grains with a fork and stir in the butter, herbs, lemon juice and salt and pepper to taste.
Make the tahini yogurt sauce. Combine all the ingredients in a bowl and season with salt and pepper. Serve with the vegetables and couscous.