Ashkelon was one of the five Philistine cities, and sits on the Mediterranean coast. Being on the coast, a lot of produce traveled through it. Of the vegetables that Ashkelon was widely known for included types of onions. It is debated whether the onions originated in Ashkelon, or were exported/imported via the city. In either case, from the name Ashkelon came two English words for those onions:
scallion: c.1300, from Anglo-Fr. escalone, O.N.Fr. escalogne, or O.Fr. eschaloigne, all from V.L. *escalonia, from L. (caepa) Ascalonia "(onion) from Ascalon".
shallot: 1664, from Fr. échalote, from M.Fr. eschalotte, from O.Fr. eschaloigne, from V.L. *escalonia
As this site writes:
The shallot (Allium cepa, var. aggregatum) is also listed in older works as Allium ascalonium after the town of Ascalon, Syria where it is said to originate. Theophrastus (372 - 288 BCE) describes the "Askolonion krommoon" which may or may not be a reference to the shallot. Pliny (23 - 79 CE) describes "the Ascalon onion, named for a town in Judaea" in the Natural History, Book XIX but this seems to be in reference to a different Allium, because it is propagated from seed rather than divisions.
Ashkelon was also described by the Greek historian Strabo:
The country of the Ascalonitae is a good onion-market, though the town is small.
In Hebrew these types of onions are known today as betzal yarok בצל ירוק - "green onion", as they are also known in parts of America. This site writes that:
Immanuel Loew, the German-Jewish scholar who pioneered the study of plants mentioned in ancient Jewish sources, designated the בצלצול betzaltzul as A. ascalonicum.
I find this interesting, mostly since we use betzaltzul as a nickname for our son Betzalel...
As far as the etymology of Ashkelon אשקלון, there are those that feel it comes from שקל shekel, but I haven't seen an explanation other than the appearance of that root in the name.