The short answer, unfortunately, is: you can't.
I hate to shoot you down, but one of the things about truly great food is that it is fleeting. It is there when you are digging in and gone when you are done . . . sometimes forever. The secret is to acknowledge it as you're taking it in, as opposed to remembering how good it was as soon as you set your fork down.
Sure, I could go rattle off a list of tangible reasons why you can't recreate your soup de poisson: It is impossible to get the same quality of the same fish, much less pair it with similar co-star ingredients. In other words, you can't tangibly recreate this soup with a facsimile of flavor.
However, even if I could secretly serve you a bowl of the exact soup that made the impression, I'm going to go out on a limb and suggest it would not strike you as the same. For more intangible reasons, such as maybe your body was short on protein, vitamin B6 or omega-3's, leaving you with a craving for fish soup. Maybe you were on vacation, you were relaxed and you were in the beautiful and romantic South of France. Inevitably your mood was affected by these, and mood has a huge bearing on how we perceive food.
Imagine a great burger after hearing a loved one has past, and a great burger on spring break with friends. Even if they are the same burger, they will taste different.
My point? . . . We taste with more than our tongues, and acknowledging that is giving ourselves the freedom really enjoy our food.
You can buy great fish here in NYC and we have access to incredible produce from countless farmers and purveyors. They won't reconstruct soup de poisson from the south of France, but they can compose a great fish soup. The eater just has to see it for what it is, and not what it's not.
So pay attention to what your putting in your mouth, and appreciate it for what it is, because you never know when you're going to eat a great memory.
