Put the butter onto a large plate. Mix the natto with the sauce and Japanese mustard included in the pack.
When the pasta has finished boiling, put it on the plate with the butter and combine well. Pour on the soy sauce and lemon juice, and stir the flavors in. Top with the natto and shredded nori seaweed.
Story Behind this Recipe
I didn't want to have a lot of dirty dishes to wash.
If you use tubed of butter, it combines well with the pasta and is easier handle. You could also switch the soy sauce with mentsuyu. Garnishing with finely chopped scallions would also be okay. It's easier to eat if you use chopped natto.