Put the aubergine in a foil-lined roasting tin, scatter with the garlic, season with salt and pepper and spray with a little olive oil. Cover tightly with foil and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 35–40 minutes or until the aubergine is tender.
Meanwhile, place a cod fillet in the centre of a piece of foil or nonstick baking paper. Scatter with a little lemon rind, lemon thyme and season with salt and pepper. Drizzle over ½ teaspoon of the olive oil, then fold the foil or paper over several times to make a small parcel. Repeat with the remaining cod fillets. Place the parcels on a baking sheet and bake in the oven 12 minutes before the end of the aubergine cooking time, until the fish is just cooked through. Leave to rest.
Remove the aubergine from the oven and place in a food processor or blender with the black olive tapenade and Greek yogurt. Blend until almost smooth, season to taste and scrape into a bowl.
Serve the cod on a bed of steamed green beans, scattered with the pine nuts, if using, and with the aubergine and yogurt purée on the side.