Translation:You have a chocolate sandwich and a fish sandwich.
My grandmother used to make me chocolate sandwiches too ! With butter, nutella (or an eastern euro version) Dalmatian fig spread and chocolat sprinkles. I added the last two myself and she actually obliged my weird taste buds. I swear its good.
Now I have to make one, my boyfriend will be repulsed ! :)
'Avec' means with, 'au/a la/aux' can appear to mean 'with' but has a far more complex meaning. It describes something that's 'in something else' or 'apart of something else' or 'to something else'. You could say 'I go to the shops with my friends' (Je vais au magasin avec mes amis), but you can't say 'I go to the shops at/in my friends' (Je vais au magasin aux amis) - which is what 'au/a la/aux' conveys. It's very hard to describe, I hope this helps.
different languages have different concepts of describing things. if in english you say "with" it doesn't mean that in another language the overall vision of a thing is the same. in this case it seems more of the "sandwich of fish" case.
like in finnish you say "saanks maksaa kortilla?"(can i pay with my credit card), which literally would mean "can i pay on my card".
and in some polynesian languages you call a dog "a bird that bites people", cause they have never seen a dog and the physical concept of the object is different to them, than to you (:
"You are having" is technically one way to translate this. And French does have have a way to exclude the other forms of present tense to specify that you are doing something right now. You would place "en train de" in front of the infinitive verb. "Je suis en train de manger." means "I am in the process of eating." But you would only use it rarely, perhaps if there was a conversational misunderstanding about the action discussed.