Using a food processor, pulse the onion, garlic, parsley, and cilantro until finely chopped. Transfer to a large mixing bowl. Add the ground beef, rice, chopped tomatoes, olive oil, spices, salt, and date syrup. Mix thoroughly with your hands or a spoon until fully combined.
Trim and hollow the potatoes, zucchinis, and tomatoes using a paring knife or a melon baller. Slice the tops off the mini bell peppers and remove the seeds. Trim the ends of the onions and boil them in a medium saucepan of water for 7 minutes. Drain, let cool, then carefully separate into layers for stuffing.
Stuff each vegetable snugly with the beef and rice mixture, pressing the filling in gently without overpacking. Arrange the stuffed vegetables tightly together in a wide, heavy-bottomed pot.
In a bowl, whisk together the tomato purée, parsley, olive oil, paprika, date syrup, salt, black pepper, garlic powder, and boiling water. Pour the sauce over the stuffed vegetables. Cover the vegetables with a sheet of parchment paper, pressing it down directly onto them. Cover the pot with a tight-fitting lid.
Place over low heat and simmer gently for 40 minutes, until the rice is nearly tender and the sauce has thickened slightly. Meanwhile, heat the oven to 355°F/180°C (convection).
Uncover the pot and transfer it to the oven. Bake for 20 minutes, or until the tops are slightly golden and the sauce is bubbling and reduced.
Remove from the oven. Let rest for 10 minutes, then scatter with chopped Swiss chard or celery leaves. Serve warm or at room temperature.