Which is a correct English cook on low fire or flame heat?

Here's why:

* "Cook on low fire" is not idiomatic in English. "Fire" usually refers to a larger, uncontrolled blaze, not the controlled flame of a stovetop.

* "Cook on low flame heat" is grammatically awkward and repetitive. "Flame" implies heat, so adding "heat" is redundant.

"Cook over low heat" is the clear and concise way to describe cooking with a gentle flame.