Frank,

I'd doubt it ... onions are day length sensitive and they want to bulb up in the spring / summer versus the much shorter days in the fall. If you started seeds now, I think at best you'd get green tops to use as scallions and maybe some very small bulbs to use as sets next year.

Normally, I start onion seeds in February / March for setting out in Mid-April in zone 5 so you should be a couple of weeks earlier. I use the same fertilizer for garlic as for onions - a mix of bonemeal and dried blood in a 1:1 ratio with some wood ashes mixed in - except for onions I foliar feed with seaweed every 3 weeks or so from a month after planting until a month before harvest just to get an edge on bigger tops / bulbs. You might also want to check out a commercial place like Dixondale to get their planting guide since it deals better with more traditional fertilizers.