Pre-heat the grill to high or light a medium fire in a charcoal grill. Combine the pomegranate seeds, coriander, fennel seeds, and sesame seeds in a mixing bowl. Season the steak generously with salt and pepper, rub thoroughly with the spice mixture, and set aside.
Cut the potatoes into 1-inch pieces, place in a large pot, and cover with cold water; add enough
kosher salt until the water tastes like sea water. Bring to a simmer over medium-high heat and simmer until the potatoes are cooked through and creamy, about 20 minutes.
Grill the steak over high heat, turning frequently to char evenly; I like to serve my hanger steak slightly on the rare side, about 8 minutes on a hot grill. Once you’ve cooked the steak to your liking, set it aside to rest.
Meanwhile, while the steak is grilling, drain the potatoes, then crush them roughly with a potato masher, mixing in the olive oil as you mash. Fold in the olives, horseradish to taste, lemon juice, salt, and pepper.
With a sharp knife, slice the steak diagonally against the grain, which makes it a little less chewy. Serve the steak on a bed of the potatoes on a large platter, and finish with a drizzle of olive oil and a sprinkling of chives.