Cut the cabbage into bite-sized pieces.

Extract the mentaiko contents and reserve a small amount for garnish.

In a microwave-safe container, add 500ml water and a pinch of salt. Break the spaghetti in half and add it to the container. Cover loosely with plastic wrap and microwave at 600W for the time indicated on the pasta package. Add the cabbage on top and microwave for an additional 1 minute 30 seconds. Drain the water.

Mix the drained pasta and cabbage with mentaiko, butter, and soy sauce. Plate the mixture.

In another microwave-safe container, combine dashi powder, 120ml water, and salt. Microwave uncovered at 600W for 1 minute 30 seconds to create dashi broth.