Why does my onion have very few, very thick layers, and a hollow center?
Was this onion by any chance purchased from a local farmer's market or grower?

I have grown onions that did this, experimenting with over-wintering onions and allowing them a 2nd growth, with onions that I thought too small to harvest in fall so let them keep going.  The experiment was a failure.  Most rotted, but some did this, produced an onion that looked fine, but few, leathery layers and hollow or growth in the middle.  Taste was fine, a bit strong, but texture was less than pleasant.  Some flies and disorders like milky rot will also hollow them, but a high share of the time will also result in goo or slime and I have not seen the thick layers associated with those conditions, but that is strictly personal experience.  And, it could easily be an individual mutant or bolted plant.  

Any stress while growing, too much or too little water, heat, light can cause a plant to bolt and go to full maturity before it should.  In you case, to my personal eye, the onion looks like one that went to full maturity rather than a disease.  We actually eat many items, like onions and garlic before they are fully mature.  In both of these items, blooms are typically removed to try to delay the plant from fully maturing.  This helps to produce bigger bulb and clove heads.  In the case of onions, if the onion fully matures, rot occurs or a product similar to what you have.  Only a guess, but what my thinking would be is an onion that was missed when top buds were removed, or one that fully matured without bothering to bloom.