The collar of a large fish is the part between the head and the body; it has a (healthy omega-3) richness. Baste with a naturally sweet and salty sauce.
If you're going to turn on the oven in August, it better be for a good reason: This sauce, made from broiled tomatoes and peppers, is tangy, smoky, juicy, and definitely worth it.
Pro tip: Viva brand paper towels are used at Rintaro specifically for insulating the cod from its salty cure and will absorb moisture without falling apart.