Heat the oven to 160° C. In a medium ovenproof non-stick frying pan, heat the cooking oil over moderate heat. Add the spring onions, ginger, and garlic and cook, stirring, until fragrant, about 30 seconds. Add the bok choy and cook, stirring, until the leaves wilt, about 2 minutes. Add the asparagus and 1/2 teaspoon of the salt and continue to cook, stirring occasionally, until the vegetables are almost tender, about 3 minutes more.
Evenly distribute the vegetables in the pan and then add the eggs, pepper, and the remaining 1/4 teaspoon of salt. Cook the frittata, without stirring, until the edges start to set, about 2 minutes. Put the frittata in the oven and bake until firm, about 25 minutes. Drizzle the sesame oil over the top.