Combine seasonings and yuzu zest in a Ziploc bag and add the mackerel. Marinate the fish for 2-4 hours.
To Make Poached Kumquats
In a small saucepan, bring water and sugar to a boil and boil until sugar is dissolved. Then add kumquats and bring to a boil then turn down the heat to simmer for 5 minutes.
To Broil
Preheat the broiler* with a rack placed about 6" (15 cm) away from the top heating element (in the middle) for 3 minutes. Line a baking sheet with foil for easy cleaning (Brush/spray the foil with oil or wrinkle up the foil so the skin doesn't get stuck). Remove the fish from the marinade and place it on the foil. Broil medium/high for 8-10 minutes, depending on the thickness of the fish, until the surface is blistered and brown a bit. You do not need to flip it. *Typical broiler setting: Low/450ºF/232ºC, Medium/500ºF/260ºC, and High/550ºF/288ºC. I use medium or high.
To Bake
Preheat the oven to 425°F (218ºC) with a rack placed in the middle. For a convection oven, reduce cooking temperature by 25ºF (15ºC). Remove the fish from the marinade and place it on a baking sheet lined with parchment paper. Bake until the surface is blistered and brown a bit, about 15-20 minutes. You do not need to flip.
To Serve
Serve immediately with the poached kumquats.
To Store
You can keep the leftovers in an airtight container and store in the refrigerator for up to 2 days or in the freezer for 2 weeks.