Allium cepa is the cultivated onion. Typically grown as a biennial, it is a perennial plant normally planted in the Autumn for harvest the following Spring or Summer without being allowed to flower. They aren't the best garden plants as the large hollow stems are easily bent and will not tolerate much in the way of wind or disturbance. The dense flower heads, normally between 2" to 3" across, are attractive to bees who collect the nectar and occasionally the pollen. Allium cepa var. aggregatum, also known as the shallot, is seen in the photo below by Travis Owen.