Preheat oven to 375 degrees.
In a large pan, toss leeks, parsley sprigs and half the lemon slices with oil, cumin and coriander; season with sea salt. Roast, tossing occasionally, until leeks are tender and start to brown, 15-20 minutes. Remove from oven and carefully pour the oil from the pan into a heatproof measuring cup.
Reduce oven temperature to 275 degrees. Season rockfish with sea salt and place over leeks in roasting pan. Top with remaining lemon slices and pour oil back over it. Roast until rockfish is just cooked through and starting to flake, 30-35 minutes.
Serve fish with leeks and lemon and top with cracked pepper and fresh parsley leaves.