Wash the rice thoroughly under cold running water in a fine-mesh sieve until the water runs completely clear, then soak it in warm water for 20 minutes before draining.

Heat the vegetable oil in a large Dutch oven or heavy-bottomed pot over medium-high heat until it shimmers. Add the lamb cubes and sear them until they develop a deep brown crust on all sides, about 8 to 10 minutes.

Stir in the sliced onions and cook for about 5 minutes until they are soft and golden brown. Add the carrot matchsticks, cumin, coriander seeds, paprika, salt, and black pepper, stirring frequently until the carrots soften slightly, about 5 more minutes.

Pour in 2 cups of the hot water, reduce the heat to low, cover, and simmer the mixture for 25 minutes until the lamb is tender. Gently nestle the whole head of garlic, cut-side down, into the center of the lamb and vegetable mixture.

Spread the drained rice in an even layer over the meat and vegetables without stirring them together. Carefully pour the remaining 2 cups of hot water over a spatula onto the rice to avoid disturbing the layers, ensuring the water level sits about half an inch above the rice.

Bring the liquid to a boil over medium-high heat uncovered until the rice absorbs most of the surface water, which takes about 8 to 10 minutes. Reduce the heat to the lowest setting, cover the pot tightly with a lid wrapped in a clean kitchen towel to trap steam, and cook for 15 minutes.

Turn off the heat and let the pot sit, undisturbed and covered, for another 10 minutes to finish steaming. Remove the lid, squeeze out the sweet roasted garlic cloves to garnish, gently fluff the rice layers together, and serve hot on a large platter.