Preheat a grill. Make sure the grate is cleaned and oiled. Combine the parsley, mint if using, lemon juice, olive oil, anchovies, capers, garlic, and pepper flakes in a mixing bowl. Season with black pepper. Set the salsa verde aside. Rub the fish with a thin layer of oil, then season all over with salt and pepper. Place the fillets on the grill skin side down and grill for 5 minutes, until the skin is lightly charred and crisp and pulls away freely (if you mess with the fish before it's ready to flip, it's likely to stick). Flip and cook on the other side for 2 to 3 minutes longer, until the fish flakes with gentle pressure from your fingertip. Serve the fillets with the salsa verde spooned over the top.