Cut the thin skin of the mentaiko and extract the contents.

In a microwave-safe container, tear the cheese slices into pieces and add milk. Cover loosely with plastic wrap and microwave at 600W for 30 seconds. Stir, cover again, and microwave for another 30 seconds. Add mentaiko and white dashi, then mix to create the mentaiko cheese sauce.

Boil water in a pot, add salt (as needed), and cook spaghetti according to package instructions. Drain the water.