Eli Zabar, the esteemed monarch of a culinary and delicatessen dynasty in Manhattan, partnered with the online newsletter platform Substack on Tuesday night to commemorate the 50th anniversary of his beloved Upper East Side cafe, E.A.T. As the evening unfolded, a lively jazz trio filled the air with soothing melodies while servers gracefully glided across the checkered floor, delivering trays of delectable mini brioche egg salad sandwiches and rich chopped liver spread on raisin nut bread.

The cafe buzzed with an eclectic mix of writers, each specializing in diverse subjects ranging from politics to romance, from gastronomy to finance. At the heart of the gathering was Mr. Zabar, 81, who, with a twinkle in his eye, snatched a mini egg salad sandwich from a passing tray, conducting a taste test for quality assurance.

In a nostalgic tone, he shared his reasoning for hosting this celebration with Substack, reflecting on the cafe’s vibrant history, which dates back to a time before a turkey club sandwich at E.A.T. commanded a staggering $32. “In those days, this part of the Upper East Side was an uncharted territory,” Mr. Zabar recounted. “My initial patrons were a creative tapestry of writers, artists, and gallery owners—luminaries like Richard Avedon, Wayne Thiebaud, Leo Castelli, and Nora Ephron.”

He continued, reminiscing about legendary figures such as William Shawn and Lillian Ross, who frequented the cafe every afternoon. “They would occupy a corner table for hours, engaged in deep conversation. I affectionately referred to them as the Jean-Paul Sartre and Simone de Beauvoir of our little haven.”

As he surveyed the lively crowd gathered that evening, Mr. Zabar noted, “This assembly reminds me so much of the vibrant personalities I welcomed when I first opened my doors.”