A sheet of polythene with holes or crosses cut in it @ 5 inch intervals either way will allow you to push your sets into the soil below, but prevent weeds in between. Onions have quite a long gestation period and weeds will be prolific long before the crop is ready to be lifted. Because onions sit more or less on top of the ground, pulling weeds will tend to disturb the onions too.
I've tried this method successfully for two years now with polythene one year and an old cast off door mat last year. This year I'm trying a length of roofing felt, so we'll see.