Add tomatoes and lamb with any accumulated juices.
Bring to a boil.
Add 2 1/2 cups stock.
Return to a boil, reduce heat to low, partially cover, and simmer, stirring occasionally, until lamb is starting to get tender, about 1 hour 30 minutes.
Add the chickpeas and continue to cook for 30minutes or more until the Lamb is tender.
Stir in apricots; simmer until heated through, about 5 minutes. Season with salt and pepper.
Spoon couscous onto a large, shallow platter, forming a large well in center. Spoon tagine into center. Sprinkle cilantro over.
For the Ras-elHanout
Toast coriander and cumin seeds in a small skillet over medium heat, stirring occasionally, until aromatic and slightly darkened, about 4 minutes.
Transfer to a spice mill; let cool.
Add crushed red pepper flakes.
Process until finely ground.
Transfer to a small bowl.
Add remaining ingredients; whisk to blend.
DO AHEAD: Can be made 1 month ahead. Store airtight at room temperature.