Thinly slice on an angle some of the green parts of spring onion (to yield about 2 tablespoons) and set aside. Chop the remaining spring onions into 5 to 6 cm lengths and add to a small saucepan. Add the oil to the saucepan and place over a low heat for 30 minutes to infuse.
Strain the oil, discard the onions and allow the oil to cool slightly before adding the soy sauce and kecap manis. Add the sauce to a large mixing bowl and set aside while you steam the bok choy and noodles.
Lightly oil a bamboo steamer and add the bok choy and noodles.
Add the bok choy and noodles to the boil with the oil and toss to combine. Add the salt and black sesame sees and toss through to coat.
Divide the between 4 bowls and garnish with the reserved spring onion.