Place lamb in a large bowl and sprinkle with salt, coriander, cumin, cayenne (if using) and black pepper. Stir to coat. Let stand at room temperature for 1/2 hour.
Heat oil in a heavy skillet over medium heat. Add mustard seeds and cover to avoid splattering. Add onion and ginger and cook until onion is lightly brown.
Add lamb and its marinade, and mix with onion. Stir for about 5 minutes. Stir in water and bring to a boil. Cover, reduce heat and simmer 30 - 45 minutes. Do not stir. Lamb should be tender when pierced with a knife.
The sauce should be thick. If needed, increase heat and cook until sauce thickens.
Sprinkle with lemon juice and coriander (if using). Stir gently.