What Is This Mystery Fruit?

(Image credit: Kathryn Hill)

We saw this mystery fruit at the Alemany Market last weekend. Does anyone know what it is?

The farmer who was selling it didn’t speak much English, so we didn’t quite understand what he said. This fruit is round and green. The rind is firm but not hard; the shell can be pierced with a fingernail. Inside, the shell is about 1/6 of an inch thick, and white. There are white seeds inside. It’s got the same color and texture as a cucumber, but isn’t a cucumber. The smell is fresh and grassy.

Does anyone know what this is?

(Image: Kathryn Hill)