Mutton was the meat of mature sheep or goats that was used as food.
In 2367, Miles O'Brien expressed his intention to introduce Keiko O'Brien to several foods he remembered from his childhood, including mutton shanks. (TNG: "The Wounded")
In 2376, mutton was one of the meals that Neelix wanted to prepare in honor of Tom Paris' popular holoprogram Fair Haven. (VOY: "Fair Haven")
In 2381, Lieutenant Commander Andy Billups explained to his mother, Queen Paolana, that the reason he chose Starfleet over his birthright as the Hysperian prince was because "[he didn't] want to live in a castle with pet dragons and all the mutton [he could] eat." (LD: "Where Pleasant Fountains Lie")