1 cup yogurt, preferably full-fat
Salt and pepper
Large pinch Aleppo pepper or small pinch cayenne
1 shallot, peeled and minced
1/2 teaspoon saffron threads
Juice of 1/2 lemon, or more to taste
In a small bowl, whisk yogurt together with salt, pepper, Aleppo pepper or cayenne, and shallot. Rub saffron between your fingers to crush it, then stir it into yogurt mixture. Let sit for about 20 minutes (or covered and refrigerated for up to 2 hours). Just before serving, add juice, taste, and adjust seasoning.
Sprinkle halibut with salt and Aleppo pepper or cayenne. Put butter in nonstick skillet large enough to hold halibut, and turn heat to medium. When the butter melts, add fish, and cook gently, turning once or twice, until a thin bladed knife meets little resistance when inserted into the thickest part, generally less than 10 minutes.
Serve fish hot, warm, or at room temperature, with the sauce spooned over it.