Peel the onions and cut a slit to the center of each. Place in a large pot of boiling water and cook for 15 to 20 minutes, until softened. Drain in a colander and transfer to a bowl of ice water to cool quickly.
While the onions cook, stir together the rice, cilantro, parsley, tomato, olive oil, lemon juice, salt, pepper, cumin, and garlic powder. Carefully separate the onion layers. Fill each layer with a spoonful of the rice mixture and roll tightly.
Preheat the oven to 355°F/180°C (convection), and arrange the stuffed onions snugly in a 9x13-inch/23x33 cm baking dish.
In a bowl, whisk the boiling water, date syrup, and salt, then pour over the onions. Cover tightly with aluminum foil and bake for 2 hours. Uncover and bake 15 to 20 minutes more, until lightly browned.
Let rest for 10 minutes before serving warm.