Cook potatoes in boiling water until tender throughout when pierced
(about 20 minutes). Drain and let cool briefly; then coarsely chop.

Melt 1/2 cup of the butter in a wide frying pan over high heat.
Add potatoes and cook, stirring often, for about 5 minutes;
add more butter if needed to prevent sticking.
Sprinkle curry powder and mustard seeds over potatoes
and stir until blended. Continue to cook,
stirring often, until potatoes are lightly browned.
Stir in onions and cook, stirring, until onions are warm.
Season to taste with salt.