1. Heat oil in a frying pan over medium heat. Add mustard seeds and cook until they pop, 2 to 3 minutes. All at once, stir in remaining spices and 1 1/2 tsp. salt; cook until fragrant, 30 to 45 seconds. Remove from heat and whisk in 1/2 cup yogurt.
2. Stir 3 tbsp. spice mixture with remaining 3/4 cup yogurt in a bowl; chill. In a shallow bowl, combine remaining spice mixture with lemon juice and 3 tbsp. water. Let cool. Add salmon, turn to coat, and chill, covered, at least 1 1/2 hours or overnight.
3. Heat grill to medium (350° to 450°). Transfer salmon to a plate. Turn potatoes in marinade. Let reserved yogurt sauce stand at room temperature. Spray 2 perforated grill pans* with cooking oil spray, then arrange potatoes in a single layer over 1 1/2 pans, spray them with oil, and spray salmon. Reserve remaining marinade.
4. Grill potatoes, covered, until browned on underside, 15 minutes. Turn over, brush with remaining marinade, and cook until second side is brown, 5 to 8 minutes; as done, transfer to a bowl and keep warm. Meanwhile, set salmon oiled side down on empty half of grill pan; cook, turning once, until just cooked through, 5 minutes total.
5. Toss potatoes with onion and chopped cilantro, then spoon onto plates. Add salmon and garnish with cilantro sprigs. Serve with reserved yogurt sauce and add more salt to taste.
*Or fold 2 sheets of foil in half. Poke dime-size holes in them about 2 in. apart, using a small knife.