grammatical number - Was vs were in "Today's lunch was sandwiches"

I was told the sentence "Today's lunch was sandwiches", should be "Today's lunch were sandwiches", but I don't think this is correct.

However, when I switch the order I end up with this:

Sandwiches were today's lunch.

Therefore, because "were" seems appropriate here, I thought maybe "were" is correct in the above example as well.

However, no matter how many times I try to convince myself, "Today's lunch were sandwiches" just sounds wrong.

Is there ever a case where "Today's lunch were ~" is correct?

Answer

Your problem is numerical agreement. The verb and the subject must either both be plural or both be singular.

Sandwich were today's lunch <-- this is bad! It probably feels bad the same way the 'correction' felt bad! That's because it is also wrong because 'Sandwich' and 'were' don't agree.

Sandwiches were today's lunch <-- plural and plural are fine, this sentence is good.

Today's lunch was sandwiches <-- this is fine. Lunch is singular and so is was.

Today's lunches were sandwiches <-- this is ok too. Both of them are plural.