Planting Onion Sets

I tried growing onions from seed this year and I have really nice looking grass.  Basically I think I should have started them last year if I wanted to transfer them to the garden this spring.  I was at Tractor Supply picking up bird feed yesterday and bought 240 onion sets, 3 bags of 80.  I ran some mason twine at 4" spacing and planted them 4 inches apart.  I was 8 short, so I have a nice bald spot right in the middle.  Some kind of flower will probably go there.