I can’t think of a better match for this delicate but rich-tasting sauce than firm-fleshed halibut fillets. Serve the curry over a bed of slightly sticky long-grain white rice to mop up all the succulent flavors.
Sprinkle the turmeric over both sides of the halibut fillets, gently pressing the yellow spice into the flesh. Refrigerate, covered, for about 30 minutes.
Heat 1 tablespoon of the oil in a small saucepan over medium-high heat. Add the mustard seeds, cover, and cook until the seeds have stopped popping (not unlike popcorn), about 30 seconds. Add the lentils and sti