Cut the cabbage in half horizontally and slice into 2cm wide strips.

Make a slit in the mentaiko and extract the contents.

Boil water in a pot, add salt (1 tsp, not listed in ingredients), and cook the spaghetti according to package instructions. Add the cabbage 1 minute before the spaghetti finishes cooking. Drain well.

In a bowl, combine the mentaiko, butter, soy sauce, and the drained spaghetti and cabbage. Mix well.