Preheat the oven to 220C/440F
Cut the eggplants lengthways through the stock, score the insides with a sharp knife and brush with 1tsp of olive oil each, salt and arrange on a baking sheet cut side up. Cook in the oven for 30 minutes until golden and the flesh is easily scoopable.
Meanwhile cook the beef filling. In a large pan heat the olive oil and cook the onion over low heat for 5-7 minutes, add garlic and cook for 1 minute longer without allowing the garlic to colour, otherwise it will taste bitter.
Add the beef and brown until no longer pink, then add cumin, cinnamon, tomato paste, 1 or 2 tbsp of harissa paste, depending on how spicy you like it. Stir everything together, then add stock or water. Salt the beef mixture last as harissa paste and stock will have various amounts of salt.
Bring the beef to a boil, then lower the heat and let it simmer for about 10 minutes until most of the liquid is cooked away but the mixture is not dry.
When the eggplants are cooked, let them cool slightly, and then remove the flesh. Cutting alongside the edge of the eggplant while leaving a 2 cm border makes it easier to do. Chop up the eggplant flesh and add it to the beef, stir to combine.
Stuff the eggplant with the beef mixture and cook in the oven at 180C/350F for 10 minutes.
Remove from the oven when piping hot. Drizzle with plain yogurt and sprinkle with chopped parsley before serving.
You can serve with additional yogurt sauce and a harissa paste swirled into. (If your harissa paste is too thick, thin it out with water.)