Heat your oven to 200°C/Fan 180°C/Gas 6. Halve the aubergines lengthways through the stalk. Using a sharp knife, make a criss cross pattern in the aubergine flesh. Take care not to score all the way through to the skin. Place on a baking tray.
Add ½ tsp harissa to a bowl. Pour in 1 tbsp oil with a pinch of salt. Stir together. Brush this spicy paste all over the flesh of the aubergines.
Peel and thickly slice the onion. Tumble onto the baking tray alongside the aubergines. Tip on the baby plum tomatoes. Season both with salt and pepper. Slide the tray into the oven and roast for 30 mins.
After 30 mins, remove the baking tray from the oven. Spoon the tomatoes into a bowl, leaving the other veg on the tray.
Sprinkle 1 tsp of the turmeric into the bowl of tomatoes. Using a fork, squash everything together to form a rough paste.
Spoon the yogurt into the bowl of tomatoes and mix together to make your smashed tomato sauce. Taste and add more salt and pepper if it needs it.
Scatter the rocket onto two plates. Drizzle over ½ tbsp oil. Top each with two aubergine halves and slices of roast onion. Spoon over the smashed tomato sauce and serve, sprinkled with a little extra harissa if you like.