I heard this and knew I HAD to share it! A euphonium handling the solo part of Mozart's Clarinet Quintet (solo plus 4 strings). Good job by Cynsy Plant on the solo!
(I love the creativity of seeking this type of repertoire. Somewhere I have an amazing recording of Roger Oyster playing the Weber Concertino for Clarinet with the St. Louis Symphony. As I recall he played this with piano for a competition at ITEC 1978.)