Prepare the vegetables: wash and hollow out the Zucchini and the Eggplant (leave a shell about 1/2 cm thick). Cut the tops off the Bell Pepper and remove seeds and membranes. Slice the tops off the Tomato and scoop out the pulp into a bowl (reserve the shells), and blanch the Grape Leaves briefly if using. Peel and slice the Potato into rounds to line the bottom of the cooking pot.
Rinse the Egyptian Short-Grain Rice under cold water until the rinse runs clear, then drain well.
Make the filling: heat a few tablespoons of Vegetable Oil in a pan over medium heat. Add finely chopped Onion and sauté until translucent, then add minced Garlic and cook for another minute.
Stir in the drained rice, the reserved tomato pulp, and a spoonful of Tomato Paste. Add chopped Dill, Parsley and Cilantro, then season with Ground Cumin, Ground Allspice, Dried Mint, Salt and Black Pepper. Cook the mixture for a few minutes so the rice is well coated and the flavors meld; remove from heat and let it cool slightly (the rice should be only partially cooked by steam later).
Fill each prepared vegetable loosely with the rice mixture (do not overpack, the rice will expand). Fill the Tomato shells with the same mixture as well, and fold any extra grape leaves around stuffed items if desired.
Arrange the sliced Potato rounds in the bottom of a wide, heavy pot to protect the stuffed vegetables from direct heat. Stand the stuffed vegetables upright and snugly together on top of the potatoes; tuck any stuffed Tomato and wrapped Grape Leaves around them so nothing floats.
Make the cooking sauce by combining Tomato Passata with enough Water to almost come up to the shoulders of the stuffed vegetables, stir in a little more Tomato Paste or reserved pulp if you like a richer sauce, a splash of Lemon Juice, a drizzle of Vegetable Oil and a pinch of Salt and Black Pepper. Pour the sauce gently into the pot around the vegetables.
Place a small heatproof plate on top of the stuffed vegetables to keep them submerged, cover the pot, bring to a gentle simmer, then lower the heat and cook for 45–60 minutes until the rice and vegetables are tender. Check once or twice during cooking and add a little hot water if the sauce drops too low.
When done, remove from the heat and let the pot rest covered for 10–15 minutes. Finish by squeezing a little fresh Lemon over the mahshi and scatter extra chopped herbs if desired. Serve warm, spooning sauce over each portion.
