Dibbered leeks.

... leave the hole open and it will allow the leek to expand as it grows.

Simples.

 
Ahem, is all expansion already finished for leeks? Mine got to non-invisible seedling size this year before planting out, but most are only about as big around as a finger. I think I might have managed a few thumbs! Mine are Lyon 2, which seem to suggest they could be harvested Sept-Dec.