Is it "it" or "that"?

Experts,
Let's say that you asked a server what he'd recommend and he suggested a certain dish, and if you wanted to ask him back how that dish was cooked, would you say "How is that cooked?" or How is it cooked?”
In other words, when you are talking back and forth about a certain thing such as in the above situation, this certain thing should be referred as "it" or "that"?

Thank you.