The steak, which we had for dinner last night, was delicious.

[keannu]

My grammar book says only 1 is possible, what about 2 if considering more context?

gz46
1. The steak which we had for dinner last night was delicious.
2. The steak, which we had for dinner last night, was delicious.

 

[emsr2d2]

My grammar book says only 1 is possible, what about 2 if considering more context?

gz46
1. The steak which we had for dinner last night was delicious.
2. The steak, which we had for dinner last night, was delicious.

#2 is fine if you are giving "which we had for dinner last night" as extra but unnecessary information.
In my opinion, #1 would sound more natural if you remove "which" completely - "The steak we had for dinner last night was delicious".

 

[keannu]

You mean even if there no pre-context like the parenthesis, the underlined is possible? I don't think you can use "the" first without any description, if the description is a non-restrictive one, but I'm not sure if native speakers perceive non-restrictive clause first or later.

(I had some steak at TGI yesterday). The steak, which we had for dinner last night, was delicious.

 

[Grumpy]

It's all about the context.
First, let's suppose that you are discussing the quality of the food served in your hotel. Then you might say, "The steak we had for dinner last night was delicious".
Next, suppose someone asked you whether you had had the chance yet to taste any of the beef cuts he had given to you as a present. Then you might say, "The steak, which we had for dinner last night, was delicious".

So the first example is a general comment on the meal; the second is more specific to the steak and the circumstances in which it was eaten.