Heat the oven to 425. Place the potatoes and garlic in a cast iron skillet and toss with the olive oil, a big pinch of salt (about a tablespoon), freshly ground black pepper, and the spices. Place in the oven and roast, stirring occasionally, until the potatoes are wrinkled, the garlic is golden, and a knife pierces a potato easily. (Sometimes this takes longer, like an hour or so.)
While the potatoes roast, make the gremolata. Stir together the olives, the lemon zest, juice, capers, and parsley. Stir in olive oil (about 1/4 cup.) Taste and adjust with more lemon juice.
To finish, lay the cod filets on top of the potatoes. Drizzle with a little more olive oil, sprinkle with salt and pepper, and place under the broiler and cook until the internal temperature reaches 145.
To serve, spoon lift the cod on to plates, spoon the potatoes on the side, and top the fish with the gremolata. Eat right away.