Combine broth and pomegranate juice in a large heavy pot, Dutch oven, or tagine.
Whisk in coriander, cumin, paprika, cinnamon, ginger, 1 ¾ tsp. salt, and ¾ tsp. pepper.
Add lamb, onion, and garlic, toss to coat, and gently simmer over low to medium-low heat, mostly covered, until lamb is fork-tender, about 1 ½ hours.
Uncover, increase heat to medium, and continue to cook until juices are thickened, about 15 minutes more; taste and adjust seasonings.
Transfer lamb mixture along with any accumulated juices to a serving dish.
Top with parsley or cilantro, pomegranate arils, and mint.
Tagine can be made 3 days ahead; cover and chill. Rewarm before serving.
