Put oil and mustard seeds in a skillet, cover, and heat over medium-high heat. They'll start popping wildly. When the popping sounds die down, remove from heat. Let cool slightly, uncovered.
Halve and pit apricots; cut into 1/2-in. dice. Put diced apricots in a large bowl and toss gently with lemon juice, onion, cilantro, chile, and mustard seeds with oil. Stir in 2 tsp. salt and chill for at least 30 minutes and up to 2 hours.
Wipe skillet clean with paper towels and fill half-full with water. Add 1 tsp. salt and lemon slices. Heat water over medium heat until gently simmering. Add fish and cook, covered, until just opaque in center (cut to check), 5 to 8 minutes.
Transfer fish with a spatula to a platter and chill at least 20 minutes. Season with salt to taste and serve cold, with salsa.
Note: Nutritional analysis is per serving.