For next year, just before you expect the veg shoots to emerge, take a weed wand across the bed. This frazzles the weed seedlings but your veg are still protected by the soil.
If you're not using fleece, to tell when the veg seedlings are about to emerge, take a small piece of glass to cover a small patch at one end of the row. The instant you see your crop seedlings beginning to appear, take your weed wand over the rest of the bed. The rest of the crop seedlings will emerge a couple of days later to a clean seedbed (hand weed the bit where the glass was). This works particularly well for carrots which take a long time to emerge, so can soon be swamped by weeds.
For this year, if your pot leaves have emerged, then you can earth them up, so the leaves are under the soil. This destroys annual weed seedlings (or at least gives them a fright) and protects the shoots from frost. Then sprinkle a not too thick layer of grass clippings between the rows.
For the onions, I've found that the only safe thing to do is to hand weed.