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Onions specifically offer a great glimpse into the mechanism of this aroma giving process. Their sulfurous "bouquet" has been likened to the scent of female sweat (as has grapefruit, another sulfurous material), but no matter what your view on that is (Flaubert and Baudelaire notwithstanding), most people classify volatile sulfurous components as unpleasant, more on which later. The organosulfur compounds called thiols present in onions (allyl mercaptan is the compound released upon slicing an onion) form a group, also called mercaptans in the older days; a portmanteau deriving from the Latin mercurium captans thanks to their superior bonding ability to mercury compounds. Thiols/Mercaptans are compounds with the -SH group bonded to a carbon atom.
Now you might be forgiven to think that mercaptans remind you of decay and stench; they're produced by animal and plant decay, are found in beer that has been exposed to ultraviolet light or in faulty wines (sulfur and yeast reacting in wild patterns) and are infamously contained in skunk secretions and in flatus. One form of mercaptan, T-butyl mercaptan, is routinely added to otherwise odorless natural gas to render a leak more likely to be detected. But they're not a damning thing per se: specific forms of thiols are responsible for the characteristic and coveted scent profile of grapefruit or ~interestingly!~ released upon roasting coffee beans, surely my idea of heaven this side of heaven.
The good part involving cooks and onions is that thiols can be easily oxidized to disulfides and higher oxidation products such as sulfonic acids, free from the associations had with their predecessor. Furthermore onions are comprised of 75% water and they contain complex sugars. By browning a sliced onion in the pan the increased temperature makes water evaporate and break the bonds that hold chemical compounds contained and we see the plant matter shrink in front of our eyes and become soft and miserable. Yet those complex sugars are thus broken into monosacharides, i.e. glycose and fructose, resulting in caramelization and a more intense, sweet flavor than previously.
The so called Maillard reactions, a non-enzymatic type of chemical reaction that happens a lot in the kitchen, even at room temperature, also accounts for the breakdown of a reducing sugar with an amino acid, rendering things brown (This is the chemical process responsible for the nicely brow appearance of baked goods). The larger the sugar, the slower it'd react with the amino acids. Last but not least, the cysteine in the stewed pork (another sulfur containing ingredient) reacts with the sugars in the onions in another Maillard reaction to render the umami of meat that fills the mouth with rich satisfaction and the kitchen with the sweet caramel goodness of a deceptively wholesome dish. How far the mind can wander while making dinner…
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