This recipe uses a rice cooker. It has an extended keep warm function, which holds food at 60ºC+. Hot water at that temperature is just right for making hot spring eggs.
Place cold egg in rice cooker. Add 30ºC water to cover 40% of egg. Add 100ºC water till egg is just totally submerged. Cover rice cooker. Select extended keep warm function. Cook egg for 1 hour.
1 cold egg,
Place soba sauce in a bowl. Add hot water. Stir thoroughly.
1 tsp soba sauce, 2 tsp hot water
Crack egg into sauce. Sprinkle with spring onions. Serve immediately.