Why does a cooked onion taste sweet?

You might notice that a variety of foods taste sweeter or juicier after cooking. In heating these foods, you break down the large, complex molecules within them and create smaller molecules that are easier to digest. Your tongue is more sensitive to these smaller molecules; after all, if your body has the choice, it prefers them to their larger, less digestible analogues. Heating an onion is no different: since the onion bulb stores nutrients for the plant, it is full of chains of fructose called "fructan". When these break apart, they taste very sweet.