Onions are effectively a monopoly, the company that packages them contracts the growers before the season so while there isn't a futures market to trade there is the same. (I don't know how effective the monopoly is, could be a number of different companies that compete, but farmers don't plant onions without a contract, and even if they did there is no market if you don't have a contract so the crop is worthless.

Potatoes don't have a future market, and they are still planted. Though I believe even then big names like McDonald's contract all their potatoes in advance.