ハムとチーズが山型パンで、ほうれん草がチャバタです

Question

In this conversation (video clip):

店員：お決まりですか？
客１：私はハムとチーズで山型パン
店員：はい
客２：じゃ私はほうれん草をチャバタでお願いします。
店員：はい、ありがとうございます。

店員：お願いします
オーナー：はい
店員：ハムとチーズが山型パンで、ほうれん草がチャバタです。
オーナー：はい、お願いします。

I am not sure why が is used here. を makes sense, as in the utterance from one of the customers, since it's を＋お願いします, but why が? Why doesn't the waitress use を too? With が it sounds to me almost like metonymy--as if ほうれん草 stands in for the customer who ordered ほうれん草 or the order with ほうれん草, but if that were the case wouldn't は be a better word choice?

Answer 1

It would make more sense to use が if the main items in the order, ハムとチーズ and ほうれん草, had already been passed on to the owner through some communication system and the waitress only had to associate each of them with a type of bread orally.

（２つの内）ハムとチーズ（の方）が山型パンで、ほうれん草（の方）がチャバタです。

For the purpose of passing on the content of the order completely, the waitress should probably say:

（注文は）ハムとチーズが１つと、ほうれん草が１つです。ハムとチーズ（の方）が山型パンで、ほうれん草（の方）がチャバタです。

However, omitting the first sentence causes no information loss in practice. I guess they chose to do so in their process in order to save time or to reduce errors.

Answer 2

In both conversations, ハムとチーズ and ほうれん草 are simply a way to refer to sandwiches by its filling - I suppose you say Ham and cheese in English, too. So it can be called a metonymy but using fillings for sandwiches.

On the が, practically you can use は as well.

1. ハムとチーズは山型パンで、ほうれん草はチャバタです。
2. ハムとチーズは山型パンで、ほうれん草がチャバタです。
3. ハムとチーズが山型パンで、ほうれん草はチャバタです。

To me, in this particular context, every pattern is ok, but using は sounds a little like assuming the owner already knows that there are orders for ham&cheese and spinach, and may be less natural for that.

The usage が should be so called 排他 (e.g. this). So 3 (and the sentence in question) means 'ハムとチーズ (and not other filligs) should use white bread' (btw: does English distinguish 山型 and not-山型?). This is close enough to は, so interchangeable to some extent.