What is eating my onions?

What is eating my onions please?  They were planted last autumn from sets, red and brown onions, they’ve grown beautifully since then, but recently, something is eating them at ground level.  Half my crop has disappeared.  Whatever it is, is not interested in the stalks, just the bulbs, so all I can see is stalks laying flat on the ground, and when lifted, there’s a neat hole where the onions once were.  As if they’ve been eaten from the inside out.  This is also happening with those grown from seed.  I follow a rotation, so they’re in a different location to last year.  I see no sign of critters - tunnels etc, so I’m thinking something smaller than those?  Any thoughts with possible solutions will be most gratefully received.

Photos please. Because we do not plant onion sets the previous year. They might have just naturally died.