Cut open the mentaiko and extract the roe.

In a pan, add water and seasoning ingredients (soy sauce, dashi powder, garlic paste). Heat over medium heat until boiling.

Break the spaghetti in half and add to the pan. Cover and cook over medium-low heat, stirring occasionally, for 1 minute less than the package instructions.

Remove the lid and cook over medium heat until the water evaporates. Turn off the heat, add butter and 2/3 of the mentaiko, and mix well. Adjust seasoning with salt and pepper.