Place the aubergine halves into a roasting tin. Roast in the oven for 30-45 minutes, or until tender, then set aside.
Heat a frying pan over a medium heat until hot. Add the duck breasts, skin-side down, and fry for 6-8 minutes until the skin is crisp and most of the fat has melted (rendered).
Turn the duck breasts over and continue to fry on the other side for 2-3 minutes, or until cooked to your liking. Remove from the pan and set aside on a warm plate to rest.
Line a lidded wok with a double layer of aluminium foil. Sprinkle the wood chips into the bottom of the wok, then cover the wok with a lid and heat over a low heat, until the wood chips start to smoke. (CAUTION: Food-smoking generates a great deal of smoke. Cook in a well-ventilated place and open the windows. Open the wok outdoors.)
Add the aubergine halves to the wok, then quickly replace the lid and smoke for 3-4 minutes.
Remove the smoked aubergine halves from the wok outdoors.
Transfer the smoked aubergines to a food processor, then add 50g/2oz of the butter and all of the lemon juice and blend to a purée. Season, to taste, with salt and freshly ground black pepper.
Heat the remaining butter in a separate frying pan over a medium heat. When the butter is foaming, add the spinach leaves and fry for 1-2 minutes, stirring regularly.
Add the water and continue to cook for 1-2 minutes, or until the spinach has wilted.
To serve, carve the duck breasts into thin slices. Place one sliced duck breast onto each of four serving plates. Spoon a little of the smoked aubergine purée and the sautéed spinach alongside.