Why do cooked onions taste sweet?

Answer: Heat changes sharp onion chemicals into sweeter, milder ones

The onion has simply become more ripe — Not quite. Raw and cooked onions can be the same ripeness. What changes is the chemistry inside the onion when heat is applied.

Heat changes sharp onion chemicals into sweeter, milder ones ✓ — Correct! Raw onions contain sharp sulfur-based compounds that taste pungent and can make you cry. Cooking breaks those harsh compounds down into milder ones, while the onion’s own sugars also brown and caramelize. Same onion, very different chemistry.

The onion absorbs sugar from oil or seasonings — Not quite. Seasoning can add flavor, but even plain cooked onions taste sweeter than raw ones. The main change comes from heat transforming the onion’s own compounds, not from added sugar.