Remove skin from the salmon and cut into bite-sized dice.
To make the spice paste, roughly dice the tomato, peeled garlic and onion. Put them in a small food processor or blender along with the ginger, cumin, coriander and turmeric. Blend until smooth. This can be made ahead of time and refrigerated (as can the diced salmon).
When ready to cook, warm the oil in a medium skillet/frying pan over a medium heat and add the spice paste. Cook for a couple minutes until it darkens a little and bubbles.
Add the coconut milk and bring to a simmer then reduce the heat. Add the salmon and allow it to cook in the sauce. Don't stir too much to avoid breaking up the fish.
Add the lemon juice and a little water as needed to thin the sauce slightly - you want it to be a bit runny but not too thin. Serve over rice.