In a small fry pan over medium-low heat, toast the cumin and coriander seeds, stirring frequently, until fragrant, about 5 minutes.
Transfer to a spice grinder, add the peppercorns, paprika, ginger and the 2 tsp. salt and grind until well combined. Set aside. If using ground, skip the grinding step and just combine in a small bowl. Mix with a fork.
In a tagine over medium-high heat, warm 1 tablespoon of the olive oil until almost smoking. Working in batches, brown the lamb on all sides, 3 to 4 minutes total. Transfer to a plate. If you don’t have a tagine, we used our most favorite pasta pot.
Add the remaining oil and the onion to the tagine or pot, reduce the heat to medium and cook, stirring, until translucent, about 8 to 10 minutes.
Add the garlic and the spice mixture, reduce the heat to medium-low and cook, stirring occasionally, until the onion is caramelized, about 2 more minutes.
Add the lamb, water, potatoes and the 1/4 cup cilantro and bring to a simmer.
Cover the tagine or pot and adjust the heat so the mixture gently simmers.
Cook for 45 minutes, then add the chickpeas and lemon juice.
Continue cooking until the lamb is tender, about 45 minutes more. Taste and adjust the seasonings with salt and pepper.
Garnish with cilantro and serve. Accompany with couscous or rice.