For the sauce:
Put 2 tablespoons coconut oil into a large skillet over low-med heat and then add ginger. Saute for 30 seconds and then the fennel seeds, cardamom seeds, brown mustard, seeds and ground coriander; cook for 10 seconds. Add the tomatoes and cook for 3 minutes, stirring well. Pour in the coconut milk and simmer for about 15 minutes, until sauce is thickened. Taste for seasoning and add salt and pepper, if necessary.
For the salmon:
Heat the remaining 2 tablespoons coconut oil in a nonstick skillet over medium-high heat. When it is hot, season the salmon fillets with 1/2 teaspoon ground coriander and a pinch of salt and pepper and then sear them on each side for 7 to 8 minutes, until they are just cooked through. Put the salmon on a plate and ladle some sauce over it.