Warm the oil in a large pot set over medium-high heat, then add the onion. Cook, stirring occasionally, until golden brown, about 15 minutes.
Add the lamb, spices, bay leaf, garlic, tomato and the tomato paste, if using. Add the water, then bring to a boil. Reduce the heat to maintain a simmer; cook until the meat is tender, 30 to 45 minutes.
Add the coarsely ground wheat and si