In a medium bowl, marinate mackerel with soy sauce, lemon juice and sesame oil for at least 15 minutes. Cover and refrigerate.
In a big pan add the oil and red onion, garlic and ginger over medium high heat and saute it for 2 to 3 minutes. Then add the tomatoes, jalapeño and season with salt, pepper and fish sauce. Stir fry for another 2 minutes.
Add the red curry paste and saute until the tomatoes has completely broken down, for approximately 4 to 6 minutes.
Turn the heat into medium. Stir in the sugar and coconut milk. Cover and simmer for 10 minutes.
Uncover, stir in the marinated mackerel into the mixture carefully to prevent the mackerel from falling apart. Cover and simmer for additional 10 minutes.
Uncover, then carefully add the mustard greens on top of the mixture. Cover and simmer for 3 minutes or until the mustard greens have wilted. Stir the mustard greens into the mixture carefully and cook for addtional 2 to 3 minutes. Turn off the heat.
Best serve with steamed rice.