Heat the oil in a large skillet or wok and add the mustard seeds and wait til they pop. Add the garlic and gently cook for about 1 to 2 minutes until pale golden. Stir in the red pepper flakes and black pepper.
Add the potatoes and toss gently until they are coated and warmed through. Mixture should be very fragrant and the spices should be coating the potatoes with specs of golden brown.
Place in a mixing bowl and toss with the red onions, green chilies, grapefruit or lime juice, and black salt. Mix in the mint. Let the mixture chill for about 20 minutes before serving.
I used yellow mustard seeds as that was what I had on hand, but brown ones would work well too! I substituted fingerling potatoes for russet potatoes. I used Serrano chiles for the green chilies. I used 2 limes.