Remove the skin from the belt fish by carefully scraping it off with a knife.
Cut into 1 cm thick pieces using a strong knife like a cleaver. The bones in belt fish are quite thick to cut through. Wash thoroughly under cold water
Add 1 tablespoon of Korean sea salt and mix well. Cover and let this ferment at room temperature for 4 hours.
Wash the Korean radish thoroughly and only peel off any dark blemishes. Cut into 1 cm thick rounds, then into strips.
Add 2 tablespoons of Korean sea salt and mix well. Let this sit for 4 hours, mixing every 30 minutes or so to evenly distribute the drawn out salty water.
Cook 1/3 cup of millet and 1/2 cup water in a small saucepan for about 15 minutes on medium low heat. Set aside to cool down completely.
To make the rice porridge, mix 1/2 cup cold water with 1 tbsp sweet rice flour in a small saucepan. Cook over medium low heat, stirring constantly until it bubbles and thickens. Remove from heat and cool completely.
Mix together all of the seasoning ingredients plus the rice porridge.
After 4 hours, squeeze out as much water as you can from the radish. They should be very flexible but also still crunchy when you eat it.
Add the cooked millet and seasoning paste. Mix until fully combined, then store in an airtight container. Press down so there are no air pockets. Set on your kitchen counter for 2-3 days.
After 1 day, you will start to see bubbles on the sides. That means it is fermenting. Open the container and mix the kimchi around so the sauce is evenly distributed, and press down again. Repeat on days 2-3.
On days 2-3, check on the kimchi by smelling it. Once it smells sour enough for you, store in the fridge for up to 6 months.
I recommend waiting two weeks from the point you put the kimchi into the fridge to start eating it. If you eat it early and it seems too fishy/salty to you, don't worry! That will go away with more time/fermentation in the fridge.