Preheat oven to 350 degrees.Parboil potatoes for 15–20 minutes and then slice into ¼-inch rounds.In large ovenproof skillet, melt butter over medium heat. Sauté onion and asparagus in butter for 1–2 minutes, until slightly softened. Add potatoes in a layer and sprinkle a little salt and pepper.Whisk eggs with half-and-half and pinch of salt. Pour over vegetables in pan and let set, about 5 minutes (still over medium heat).Move pan to oven for 20–30 minutes, until completely set and slightly browned. While cooking, prepare truffle mayonnaise by whisking mayonnaise with a dash of white truffle oil.When frittata is done, flip onto plate, garnish with steamed asparagus (if desired), and serve with truffle mayo.