"On mange la sauce marron."
Depending on context, "on" can be translated by "one", "we" or "you".
I read comments from English speakers about the fact that using "one" was a bit pedantic, so I give you clues about "we" and "you".
In this sentence, the speaker is probably speaking "collectively" for himself and people around. So, you may translate by "we".
When statements are made in public, like "on doit se laver les mains plusieurs fois par jour", you will probably pick "you" since it is kind of an address, a semi-order: "you have to wash your hands several times a day". In the latter case, "you" can be either one or several persons.
The only real issue for you is to digest that "on" is masculine singular, even if meaning a collective subject.
What is "sauce marron"? It's not HP Sauce, is it? The results on Google Images look more like gravy.
I've never heard the term before, but I would say it is probably another term for espagnole sauce or brown sauce:
*Edit - wrong link for brown sauce.
I confirm there is no such thing as "the famous sauce marron", as there is the "sauce Nantua" or "sauce béarnaise", "sauce béchamel", "sauce beurre blanc"...
But I heard about a "sauce au cacao" dedicated to ostrich meat with cocoa and red wine.
Always nice to speak about food with Frenchmen/Frenchwomen ;-) One reason why I love the country and the people ;-)