Wash the zucchinis, peppers, and tomatoes well. Cut off the tops of the zucchinis carefully, keeping them aside as lids. Using a small knife or vegetable corer, scoop out the insides of the zucchinis. Cut off the tops of the bell peppers and remove the seeds. Slice off the tops of the tomatoes without cutting them all the way through, leaving them attached slightly to use as lids. Use a spoon to scoop out the flesh of the tomatoes and blend it into a purée.
Peel the onions, trim the tough stem, and make a shallow cut on one side to help separate the layers. Heat olive oil in a pan and sauté the onions until golden. Gently remove the softened outer layers and set them aside. Alternatively, boil the onions whole in plenty of water for 10 minutes until the layers soften.
Heat the olive oil in a saucepan and sauté the chopped onion. Add the grated carrot, zucchini, cabbage, and the reserved zucchini flesh. Sauté for another 7 minutes. Stir in the tomato paste and rub it against the pan to release its aroma. Add the blended tomato purée and cook for 1-2 minutes.
Add the sugar, salt, cinnamon, black pepper, cumin, dried mint, parsley, lemon juice, and bulgur. Stir for 1-2 minutes and remove from heat to allow the filling to cool slightly.
Fill the vegetables with the mixture using a spoon, making sure not to overfill them. Cover each with its respective lid and place them in a 26×38 cm baking dish or two smaller 22×32 cm dishes.
Take each onion layer and fill it with 1-2 tablespoons of the mixture, then roll them up and place them in the baking dish along with the stuffed vegetables.
Pour the grated tomatoes over the stuffed vegetables. If the tomatoes are not very ripe, mix in 1 tablespoon (20g) of tomato paste. Drizzle with lemon juice, add the vegetable stock cube if using, and pour the hot water into the dish.
Cut a sheet of parchment paper to fit the baking dish, crumple it, wet it, and place it over the vegetables. Cover the dish with its lid or aluminum foil.
Bake in a preheated oven at 180°C (356°F) for 1 hour and 20 minutes. Remove the cover and continue baking for another 20 minutes until golden and tender.