Fascinating! "Lonja" does not mean lunch at all. It is something long, wide and slightly thick, although bacon is included among those things.
I see where you found that, which is given as a possibility: http://www.oxforddictionaries.com/definition/english/luncheon
Here is another theory: http://www.merriam-webster.com/dictionary/luncheon but then they think "lunch" might have come from "luncheon" while the first think that "luncheon" came from a different word "lunch" (now extinct) with the same meaning as "lonja". Quite amusing. Then this word "lonja" 1st meaning comes from the French "longe" (2nd meaning comes from French loge, but it is not the meaning that applies here.) http://lema.rae.es/drae/?val=lonja which in turn comes from the Latin "lumbea" http://www.larousse.fr/dictionnaires/francais/longe/47749?q=longe#47671
I am wondering if that has anything to do with luncheon as it is like the game of telephone where the word changes as it is passed to each person.
Well, according to macmilan dictionary, luncheon means indeed lunch, quite formal and considering a gathering of people.
Lonja, in spanish is more about the supermarket or something like, not that intuitive even in portuguese...
We can realise the scandinavian influence in english language formation as many other english words influenced by teutonic and latin languages including french, the last tongue to "invade" GB.