Using a large chef’s knife, trim and pat the lamb slices dry. In a large sauté pan or Dutch oven, heat the olive oil over medium heat. Add the sliced onions and cook, stirring occasionally, until deeply caramelized, 25 to 30 minutes. Transfer the onions to the bottom of a tagine base or a deep 9-by-13-inch/23-by-33 cm oven-safe baking dish.

In the same pan, sear the lamb slices for 1 to 2 minutes per side until browned but not cooked through. Arrange them on top of the caramelized onions. Scatter the shallots and dried apricots around the lamb.

In a small heatproof bowl or liquid measuring cup, mix the turmeric, paprika, and pepper with the boiling water. Stir well and pour the mixture evenly over the lamb and vegetables. Drizzle the date syrup and a bit more olive oil over the top. Nestle in the cinnamon sticks and season generously with coarse salt. Cover the surface with parchment paper, then seal tightly with the tagine lid or foil if using a baking dish.

Preheat the oven to 320°F (160°C).

Transfer the tagine or dish to the oven and bake for about 4 hours, until the lamb is tender and the flavors have melded. Remove from the oven and let rest, covered, for 10 to 15 minutes before serving.

Serve warm with couscous, rice, or crusty bread to soak up the rich sauce.