Peel onions and garlic. Dice onions, press garlic through a garlic press. Wash tomatoes, dab dry and cut into small cubes. Heat 1-2 teaspoons of olive oil in a saucepan and briefly sauté half of the onions and the garlic in it. Deglaze with 1/8 litre stock, bring to the boil and remove from the heat. Stir in the couscous carefully and let it swell for about 2 minutes.
Mix in butter and tomatoes and cook over low heat for another 3 minutes. Add 40 g cheese, fold in and season with salt and pepper. Clean, wash and halve the zucchini. Hollow out the zucchini halves, sprinkle with salt and pepper. Chop the zucchini flesh. Heat 1-2 tablespoons of oil in a pot, fry the remaining onions and chopped courgettes briefly. Deglaze with remaining stock, bring to the boil briefly and season with pepper. Put the sauce in a large ovenproof dish and place the courgette halves on top. Fill the zucchini with couscous mixture, sprinkle with remaining cheese and bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 30 minutes. In the meantime, wash the basil, dab dry, pluck leaves from the stalks and, except for a few for garnishing, cut into fine strips. Mix sour cream, yoghurt, basil, remaining garlic and 2 tablespoons olive oil. Season with salt and pepper and place in a sauce boat.
Put the sauce in a large ovenproof dish and place the courgette halves on top. Fill the zucchini with couscous mixture, sprinkle with remaining cheese and bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 30 minutes. In the meantime, wash the basil, dab dry, pluck leaves from the stalks and, except for a few for garnishing, cut into fine strips. Mix sour cream, yoghurt, basil, remaining garlic and 2 tablespoons olive oil. Season with salt and pepper and place in a sauce boat. Drizzle remaining oil over it and stir lightly into the sauce. Arrange baked zucchini with some braising stock on plates and garnish with basil. Add yoghurt sauce extra