Lentils

I usually cook lentils at least twice a month. The Wikipedia says "Apart from a high level of proteins, lentils also contain dietary fiber, vitamin B1, and minerals. Red (or pink) lentils contain a lower concentration of fiber than green lentils (11% rather than 31%). Health magazine has selected lentils as one of the five healthiest foods. Lentils are often mixed with grains, such as rice, which results in a complete protein dish."

They have a short cooking time, so make sure you remember this or you'll have mush.

Some trivia: The optical lens is so named after the lentil (Latin: lens), whose shape it resembles.