Blend coriander, mint, salt, garlic, and green chillies into a paste using a food processor.
Process with lime juice, fish sauce, 1 tablespoon sugar, and 1 teaspoon ginger until smooth.
Combine the sauce and salmon in a bowl and marinate for 20 minutes.
Steam the marinated salmon in a steamer over boiling water for 6 to 8 minutes.
Blanch the pak choi in boiling salted water for 2 minutes, then drain and refresh.
Heat sesame and vegetable oils in a wok and cook garlic, spring onions, red chillies, and remaining ginger for 1 minute.
Add soy sauce, remaining sugar, and pak choi to the wok and toss to warm through.
Serve the salmon on top of the pak choi immediately.