Heat a tablespoon of oil in a pan/wok over medium-high heat. Fry ginger, garlic, and lemongrass for a minute until fragrant.
Add in the sliced onions, saute for 5 minutes until softened.
Stir in Tom Yum paste and Kaffir Lime Leaves. Cook for about 5 minutes until the colour starts to deepen.
Season with fish sauce and stir in water (adjust accordingly to your desired consistency).
Add in salmon fillets, making sure they are well-nestled in the sauce. Turn to low heat, cover and simmer for 7-10 minutes until the salmon's cooked through.
Sprinkle in spring onions and allow to cook for another 2 minutes until the spring onions start to wilt.
Garnish with more spring onions if you like and serve immediately.