So glad you asked. Do not use weed and feed in your vegetable or flower garden. You are correct in that it would not be good. As for your onions an all purpose plant food such as Osmocote would be a better choice than a high nitrogen ammonium sulfate lawn food. This is because the onions are not just making foliage like your lawn does, but also forming an underground bulb to harvest. This means the onions need more potassium and micro nutrients then a lawn and a "complete" fertilizer that contains all three of the essential nutrients (that would be the three numbers listed on every fertilizer or the N-P-K ratio) is the better choice. Too much fast release nitrogen in any garden is not good for the soil, plants or water quality. Keep growing, Marianne Binetti