Disciples of a high-fat diet will appreciate this keto-friendly sauce that comes together in a flash. (So will disciples of butter.) Think of it as a warm vinaigrette, packed with zesty preserved lemon and crunchy pistachios. The big, bodacious flavor belies the fact that it takes less than 10 minutes to make. We use it here with fish, but it goes just as well with poached or grilled chicken breast, seared scallops, and roasted cauliflower or butternut squash.