This is "A Bishop Saint and Saint Nicholas of Tolentino" by Zanobi Machiavelli. Painted in the Italian north, around 1470, it's an excellent example of a multi-panelled altarpiece from this period. As you can see, the two saints stand side by side on an angular step that encloses a patch of grass.
Saint Nicholas, the thirteenth-century mystic, wears the habit of the Augustinian Order, indicating that the original altar was dedicated to him. He makes direct eye contact with the viewer, which suggests that this panel played a significant role in the altarpiece. The gold background and decorated pointed arch are typical of Italian panel painting from this time.
But by around 1470, these features had become slightly old-fashioned, and naturalistic landscape settings were becoming more popular. This is an interesting exception to the trend, suggesting that the patron may have demanded a traditional format. Unfortunately, we don't know for certain who this bishop saint is or what church the altarpiece originally came from. Despite some speculation among scholars, it remains a mystery.
A Bishop Saint and Saint Nicholas of Tolentino hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.