The love child of this momofuku4two recipe originally taken from the Momofuku book/restaurant, and the Kimchi Udon from Ginji. I found this first iteration not as creamy as I would like, aka Momofuku version. However, the flavor was close to what I was after. I actually think there might be an egg in the Ginji dish like a carbonara, and not actually any mentaiko, but I will have to test that theory. As it is...
3 blocks of frozen udon
1/2 lb pork belly, sliced very thin
3 tbsp pork fat or butter
2.5 teaspoons kewpie mayonnaise
2 sacs mentaiko, skin removed
1/3 cup kimchi, packed; washed first if too spicy
pinch of salt
1 sheets of nori, sliced into very thin strips
2 green onions, diced

Heat a large pan or wok. Simultaneously heat water in a pot. Meanwhile, combine the mentaiko, kewpie, and kimchi in a bowl. Once the noodles go in the boiling water, very quickly start stir frying the pork belly slices. When the noodles are ready in 1-2 minutes, directly ladle the noodles into pan and remove from heat. Add butter and sauce. Toss till everything is well combined. Adjust seasonings or spiciness and serve immediately. Garnish with nori and onions.

Timing is key with this recipe. You want everything hot, the noodles slippery and chewy, the sauce emulsified, and the pork crispy. Reserve some noodle water in case you need more. Very fine pork slices match the texture of the dish better than thick. Bits of thin beef work as well. Freeze the meat for 20 minutes before slicing.