Et pour ce qui est du dîner ?

Can someone help me understand the grammar behind this sentence?  
"Et pour ce qui est du dîner ?"  In the lesson, the English translation is "and with regards to dinner?"  - -but I can't get my head around this.   It looks like, "and for what is some dinner?" to me.

I remember learning a phrase earlier  - "s'agit de" - - which sort of meant "on the subject of" I think.  Could this be used in some way here?  Or am I thinking "sur le sujet de"?  Boy, I'm confused.

Help!

Emily

I think you have the right idea with respect to the du dîner indicating a partitive. It's along the lines of "within the genre/field/scope/variety of dinners".

For the ce qui (subject relative pronoun)/ce que (object relative pronoun), it may help to think of ce qui as "what" in a dependent clause, and  ce que as "which" in a dependent clause. It's not always true, but it works about 90% of the time.

For the "structural" translation of the phrase, it's along the lines of "And which in the variety of dinner...?"  (it's an incomplete sentence)

So you're right, it functions as a "transitional" phrase in this case to change the topic to dinner choices (implies also that the previous conversation was other food choices, like for lunch). You could use « Au sujet de », like « Au sujet de nourriture » (you're transitioning the "food" conversation) but it would just be a clause, and you would still need to work in the dinner part to say something like « Au sujet de nourriture, qu'est-ce que tu préfères pour le dîner ? »

I'm not sure about the context of this sentence, but I understand how it can be confusing. In addition to torusan great answer I would add:

The du in du diner does not mean "some" as in "je mange du pain" ("I'm eating some bread", or juste "I'm eating bread"). Here it means "[Pour ce qui est de] [le diner]" ([Regarding] [the dinner], or just "Regarding dinner", "About dinner")

Sometimes, I think we just have to repeat a difficult (nonsensical) phrase until it becomes "natural" in our brains.