Drain yogurt in cheesecloth-lined sieve in the refrigerator, for at least 3 hours or overnight, discarding or drinking whey (or reserve for another use, such as bread making).

Boil potatoes; drain, peel and cut into 1/2-inch/1 cm cubes. Let cool. Stir together with drained yogurt and peas.

In nonstick skillet, heat oil over medium-high heat; add mustard, cumin and hot peppers, and fry until mustard seeds begin to pop. Immediately add onion; fry until onion just begins to soften, about 1 minute. Scrape over salad; add salt and garam masala. Mix well. Stir in coriander.

Makes 6 to 8 servings.