How to Make It
In a medium saucepan, cover the kombu with the water and simmer gently until the kombu rises to the surface, about 5 minutes. Remove from the heat. Add the dashi powder and let steep for 20 minutes. Strain the broth through a fine mesh strainer.
Return the broth to the saucepan and bring to a simmer over moderate heat. Add the soba noodles and the sesame oil and cook for 3 minutes. Add the bok choy and submerge them in the broth with a spoon. Crack one of the eggs at a time into a small bowl and gently tip it into the warm broth. Poach the eggs until the whites are just set and the yolk are still runny, about 4 minutes. Remove from the heat.
Using a slotted spoon, transfer the eggs to bowls. Stir the scallions, soy sauce, lime juice and crushed red pepper into the broth. Transfer the soba noodles and bok choy to the bowls, pour the broth on top and serve immediately.
Dashi powder is available at many health food stores and at Asian markets.