Cut the salmon into smaller pieces, to a size of your choice. I prefer rectangular pieces roughly 1.5x3 inches.
Pour the mustard seeds in a mixer/grinder jar. Add ½ cup water, 2 green chillies, ½ teaspoon cumin seeds (optional) and a pinch of salt to the seeds. Grind into a fine paste.
In a deep and big pan, heat 2 tablespoons mustard oil. Add the onions and cook for a minute or two.