Combine all the ingredients in a bowl and let sit in the fridge for 12-24 hours.
Serve on rice, sushi, etc.
Notes
If your salmon roe came in the skein (egg sac), submerge it in warm water and peel off as much of the membrane as you can. I recommend doing this over a sink. Drain and place in a bowl.
You can substitute sake for the dashi if you don't mind the alcohol. Sake gives the Ikura a milder flavor. You will need to simmer the brine in a saucepan if you use this method until the alcohol burns off (about 2-3 minutes). Cool it to room temperature before using it as a brine.
Don't let the salmon roe contact any metal as it can cause it to take on a metallic taste. I recommend using a glass bowl/jar for storage.