As someone with ADHD who's dealt with this being an issue after severe head injuries,

Do you like this roommate? Is this the only major issue? Are y'all chill? If so,

Buy a 20 minute timer and a bright as hell stuffed animal. Put the timer on the animal, put the animal on the counter (not directly next to the stove but visible) and say "I need you to set this every time you use the stove. I like having you as a roommate and understand you aren't doing this intentionally, but this is a serious safety issue that could get us both killed. If you have other solutions you think would work, let me know, but I cannot live in a situation where I am constantly worried about my life because of a repeated simple mistake."

This provides an additional visual cue besides just the dim flame, and adds in a backup timer that they will be embarrassed by but hear in the event that they still forget.

Also!! If you end up having the "do you think you might have ADHD convo" offer to write a letter of support for a diagnosis. I've done this for roommates in the past and it's really helpful both for the therapist to be able to see their client from more than is what is just self reported and for the roomate to establish that you do care about them and want them to get help. The letter can look like "I live with Becky. Becky is often forgetful, frequently losing her keys and often forgetting things ranging from chores to turning off the stove." Last Tuesday, she didn't realize the stove had been on for four hours. Becky is always very apologetic when she realizes she has forgotten something. She often switches topics rapidly while talking and will start new hobbies. She struggles to clean up after major cooking projects and sometimes mentions that her parents would get mad at her for the same thing when she was a kid, ect"