This is a variation of “Tuna and natto” or マグロ納豆. After I removed chiai 血合い from the tuna block we got, instead of throwing it out (I never tasted chiai of tuna in Japan but I never got to clean the tuna block, either), I usually make this dish or a type of soup with grilled scallion. Chiai is very dark red and has a very gamy strong taste and is certainly the least desirable portion of tuna.
After removing the chiai, I cut it into small chunks and marinaded it overnight in the refrigerator in a mixture of concentrated noodle dipping sauce (2x concentrated) with 1/3 the amount of sake (or a mixture of mirin, sake, and soy sauce 1:1:2 ratio). After soaking overnight or for 24 hours, I drained the marinade and placed the cubes of marinated chiai in a bowl. I prepared one small package of natto (for two servings) in my usual way but this time I added an egg yolk (of course, I used a pasteurized shell egg especially since we are in the middle of a big egg recalls due to Salmonella contamination). The addition of the egg yolk gives a very nice and much richer taste and texture. It is still “slimy” or becomes even slimier. (Great dish for Japanese in August)
This is usually not for Westerners but my wife likes it (it did take a long time before she could eat let alone enjoy natto). This dish needs cold sake to wash it down.