Sumac-Seared Scallops with Mostarda

I spent four wonderful summers celebrating the Fourth of July at the Pines on Fire Island, in New York. Michael and I would share a house with some of our closest friends and spend a week on that little oasis. There was only one grocery store near us, and one evening, I ended up with a bunch of scallops but no lemon or lime juice with which to flavor them. Thankfully, sumac came to my rescue and no one noticed the absence of fresh citrus at dinner. Serve these scallops hot