Set your oven to 400⁰F/200⁰C, combination steam setting. If your oven has variable steam settings, use 70-80%. If not, don't worry! Just set to combi steam at the correct temperature and the oven will figure out the humidity for you.
Cut the aubergines in half lengthways and deeply score the flesh of each in a criss-cross pattern (be careful not to cut through to the bottom though). Put them cut side up onto a solid baking tray (I use both of my solid steam oven trays as fitting them all on one is usually impossible), and cook for 25 minutes – the flesh should be soft and collapsing. Turn your oven off for now.
Let the aubergines cool slightly, then scoop the flesh into a bowl (taking care not to tear the skins, although you can always squash them together a bit later if that happenand set aside, leaving the skins in the trays.
Heat a frypan over medium heat and add the oil and onions. Cook, stirring occasionally, until the onions are soft and just starting to take on colour, then add the garlic, mince and spices. Cook, breaking up the mince for a couple of minutes, then add the harissa, tomato passata and reserved aubergine flesh. Continue cooking for about 5 minutes, mashing up any big pieces of aubergine with the back of your spoon, until the mince is cooked through. Stir in the lentils and shredded mint and remove from heat.
Turn your oven back on to 180⁰C, combination steam setting. If your oven has variable steam settings, use 70-80% again.
Fill the aubergine skins with the meat mixture, then sprinkle the feta over the top and cook to heat through and brown the cheese, about 20 minutes. Top with pomegranate seeds, if using, and a few extra mint leaves. Add a spoonful of Greek yoghurt and serve warm.