If using mallow, finely chop the leaves. If using chard, separate the stems from the leaves, and finely chop each separately.

Heat the oil in a large Dutch oven over medium heat. Add the onion and the chard stems (if using). Cover and sauté until slightly softened, 10 to 12 minutes.

Uncover and increase the heat to medium-high. Crumble the ground meat into the skillet, stirring and breaking it up with a wooden spoon, until browned, 8 to 10 minutes.

Add the chopped greens, cover and cook until wilted, about 5 minutes. Add the 3½ cups of water and bring to a boil. Reduce the heat to low, cover and cook until very soft, 45 to 50 minutes.

Stir in the salt, pepper, and cumin (if using). Use a wooden spoon to make a well in the center of the stew. Add the rice and gently shake the pan to make sure it is covered by the cooking liquid. Cover and cook until the rice is tender, about 20 minutes.

Just before serving, mix the rice into the stew and adjust the seasoning. Serve immediately with matzo pieces on the side to add to the stew — eat quickly before they soften.