Prepare a grill for medium-high heat cooking.

Place mustard seeds in a small heavy skillet and cook, shaking skillet frequently, until seeds are toasted and fragrant, about 3 minutes; you can do this on the grill grate or on the stove top over medium-high heat.

Cool the seeds slightly, then pulse them in a spice grinder until crushed.

Transfer to a large bowl and stir in tamarind, cilantro, 1 tablespoon of the oil and 1/4 teaspoon of the salt. Set aside.

In another bowl, toss shrimp with remaining 1 tablespoon oil and 1/4 teaspoon salt.

Thread shrimp on 6 (12-inch) metal skewers or place them in a grill basket.

Grill, turning once or twice, until shrimp are just cooked through, 4 to 5 minutes total.

Add to the bowl with mustard seed sauce and toss to coat. Serve hot.