The first grocery store I ever worked at my father, ever the wag, called "Grand Onion" for "Grand Union," and he wasn't far off.
Like every other (well, each of the other two) food items, the English was cheerfully stolen from Middle French. Ah, the Norman invasion. Anglo-French (and Mod. French) oignon gives us our word, and comes from the Latin unio(n). Yes, it means "union, single thing," a reference to that big unified bulb.
Bah: unlike with quince (I hope) the allium spices have already been covered better in more excruciating detail than I ever would have bothered with. With quotes from Homer, and the Greek borrowings into South Slavic, and the connections between Latin cepa and many of the European words for "onion" [and English "chive"!], proceeding on to other pages on leeks and garlic (Old English gār.lēac "spear.leek") and cloves and going on into the cloves the spice and it's all just wonderful.
So wonderful I have nothing else to add and throw up my hands at my own modest efforts. I hope, at any rate, that the link provides as much fascinating clicking for anyone else as it provided me. Quick, I better distract everyone with a pre-prepared piece of Čapek.