What is the grammar of: "You being the fish"?

Question

Source: The Big Bang Theory transcript season 2, episode 1

I found it weird when I heard it in a tv series. 'Being' is not preceded by a 'to be verb' and also comes after a subject.

How is it possible? Shouldn't it be "You are being a fish"?

Leonard: Okay, so, she said she wants to slow things down. It’s like saying “I’m really enjoying this meal, I’m going to slow down and savour it.”
Howard: No, it’s like “this fish tastes bad, so I’m going to slow down and spit it out.”
Raj: You being the fish.
Leonard: I'm not the fish.

Answer 1

This is an interesting use of the participle.

The participle in an absolute construction expresses something that exists or obtains during or throughout the time of the event in the matrix clause:

We will rob the bank just before closing time, you distracting the guard's attention.

She fell asleep, ambient music playing on her iPod.

It is this temporal aspect of "throughout" or "during" which is brought to the fore by the choice of the -ing form over a tensed verb.

You being the fish is an absolute adjunct to Howard's little analogy about what she meant by "slowing things down".

You being the fish means "Throughout Howard's analogy, the 'fish' is you" or "In this analogy we've been listening to, the 'fish' is you".

Answer 2

Being is the verb to-be, in -ing form.

You being the fish in the school play tonight made me laugh.

This is equivalent to

That you were the fish in the school play tonight made me laugh.

Note that you being does not have tense. The time has to be determined by the context and the speaker's intent.

You being the fish tonight at the school play will make me laugh

refers to future time.

You being the fish tonight at the school play makes me laugh

refers to the present.

In the above three examples, the time of you being comes from the tense of the main verb: made, will make, makes.

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Now that the context is in the main question...

Leonard: Okay, so, she said she wants to slow things down. It’s like saying “I’m really enjoying this meal, I’m going to slow down and savour it.”
Howard: No, it’s like “this fish tastes bad, so I’m going to slow down and spit it out.”
Raj: You being the fish.
Leonard: I'm not the fish.

Here, Raj just makes it clear that Leonard is the fish in this analogy.

You being the fish = you are the fish

except that the present participle provides a sense of duration. That is, it can be represented as ~~~~~~~~~~~~~~. It also indicated limited duration, so Lenard being the fish lasts only as long as the analogy is at play, that is, as long as the guys talk about this subject.