Final answer:
Onions are grilled until caramelization occurs, resulting in a sweet flavor and golden-brown color due to the Maillard reaction, while reducing the strong pungency caused by propane-1-thiol.
Step-by-step explanation:
When one grills onions, they should observe the characteristic of caramelization, which is a result of the Maillard reaction. This reaction occurs when the sugars in the onion break down and brown, giving the onions a sweet flavor and a golden-brown color.
The strong odor from onions is due to propane-1-thiol (CH₃CH₂CH₂-SH), which is a lachrymator, meaning it causes the eyes to tear due to irritation. Caramelization reduces the pungency and enhances the sweetness of onions. This makes grilled onions a popular topping in many dishes.