An elimination diet is a protocol where you start with a baseline of "safest" foods and exclude everything for a while, and then.. in an orderly fashion, start to re-introduce foods while noting how you respond or change in light of those additions. It's a protocol, one you can get from a nutritionist or dietician or by searching online. And the food lists can be adapted to people with varying eating modalities.

To clarify, the thing one eliminates is typically a molecule or a food group--the most common are probably (in no order) added sugar, gluten, fried foods, processed foods, dairy, and red meat. One can eliminate any or all of those, typically for a month (gives your gut time to change), and then, if you want to, experiment with adding things back in.

A nice thing about the gut microbiome is that it's self-reinforcing, both psychologically and gustatorily--as in, once one's body realizes a food doesn't help, one tend to both think "why would I make myself feel like that?", and to find the experience of eating/drinking it less pleasurable, alien, or even disgusting.