When Kimberly Charles came back from a business trip to Italy last fall, she brought a five-ounce, $500 white truffle home with her. She showed it to her boyfriend, Killian MacGeraghty, and described how she would make fettuccine with butter-and-Robiolo cheese sauce and carefully shave the truffle on top.

Mr. MacGeraghty didn’t share her enthusiasm. “It smells like bat-wing dust,” he told her. “I was a bit crestfallen,” says Ms. Charles, 48, the owner of a marketing firm in San Francisco. …

Ms. Charles snuck bits of the truffle into Mr. MacGeraghty’s rice and scrambled eggs. (“Did you taste that interesting, earthy note?” she asked him. Why yes, he did, he recalls. “It’s truffle!” she announced delightedly.)