View photos of the the edible cactus Echinocereus engelmannii (Hedgehog cactus), profiled in Sonoran Desert Food Plants.

A passage from Sonoran Desert Food Plants:

The fruit is the most reliably edible part: the inner pulp is pleasantly sour–sweet and very palatable. Simply slice the fruit in half, scoop out the pulp and small crunchy seeds, and consume raw.