Peel and cube potatoes into 3/4-inch pieces. Set aside.
Heat oil in a non-stick skillet over medium-high hat. Add mustard OR cumin seeds. Fry for a few seconds until mustard seeds pop or cumin seeds are roasted.
Add onions and fry until golden brown. Add garlic, curry paste and salt; fry for a few seconds. Add the potatoes. Cook for about 5 minutes, lifting and turning potatoes with a wide spatula. Reduce heat to medium and cook the potatoes for another 8 to 10 minutes, stirring occasionally.
Transfer to a serving dish. Garnish with cilantro and serve warm.