Heat the oil in a wide, heavy-bottomed pot over medium heat. Add the sliced onion and cook, stirring occasionally, until deeply golden, about 12 to 15 minutes.

Add the chicken pieces and marrow bones to the pot. Sear them well on both sides until lightly browned, about 6 to 8 minutes.

Stir in the tomatoes and garlic. Sauté together for 3 minutes.

Season with salt, black pepper, and date syrup. Pour in enough water to nearly cover the chicken. Bring to a boil, then lower the heat to medium-low. Cover and simmer for 40 minutes.

Peel the potatoes and boil them in a large pot of salted water for 7 minutes. Drain using a colander and let dry thoroughly.

Cut the parboiled potatoes into medium cubes. Heat a deep pan or pot with oil for frying. Fry the potato cubes in batches until deeply golden and crisp. Drain on a paper towel-lined plate or colander.

Add the fried potato cubes to the chicken. Gently shake or stir the pot, then simmer together uncovered for 10 minutes.

Transfer everything to a large baking dish or oven-safe Dutch oven. Bake in a preheated 350°F (180°C) convection oven for 20 minutes, or until the top is golden and bubbling. Serve hot.