Put the potatoes in a roasting pan or on a baking sheet. Drizzle with 2 tablespoons of the oil and toss to coat. Sprinkle with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper. Roast for 20 minutes. Stir and roast for 10 minutes more.
Meanwhile, rinse the cod fillets and pat them dry with paper towels.
After the 30 minutes of roasting, move the potatoes to the sides of the pan and place the fillets in the center. Drizzle with the remaining oil and sprinkle with the remaining salt and pepper, adding the red pepper flakes if using. Redistribute the potatoes around the fillets. Add garlic if using at this point. Stir.
Distribute oil (in pan with potatoes) over cod filets on both sides using a paint brush or paper towel, adding lemon pepper seasoning on both sides.
Roast until the fillets are the same color throughout and flake easily, about 10 minutes. Top with the lemon zest and chives.
Tip: Serve this roasted version of fish and chips with ketchup for the youngsters and malt vinegar for the party of adults!