Preheat a fan oven to 190°C (400°F).
Pierce the aubergines three or four times, then brush with the olive oil and arrange on a large baking sheet. Roast in the oven for 45 minutes, or until the aubergines are completely soft.
Mix the yoghurt, lemon zest, lemon juice, garlic and a pinch of sea salt in a medium bowl. Toast the pine nuts in a small, dry frying pan over a high heat, stirring for a couple of minutes.
Melt the butter in the microwave in a microwaveable bowl in 10-second bursts, then stir through the harissa. Set aside.
Cut the aubergines in half, opening them up like a baked potato. Arrange them on a warm platter and season the inside of each one with salt and pepper.
Spoon the yoghurt mixture into each of the aubergine halves, then drizzle over the harissa butter. Finally, scatter over the mint, toasted pine nuts and sliced chilli, and enjoy.