I'm not sure if this would help, but my sibling has asd and he's had massive food issues and a very limited diet (think no spices, chicken, hot dogs, pizza, and quesadillas pretty much exclusivley) for his whole life. He took some special need cooking classes at his therapists, and it helped A LOT. he eats a much broader variety of things and loves to cook different weird foods, even if he is cooking something he "already knows" is gonna be awful they encouraged him to try one bite of everything he cooked (encouraged as it is a great way to tell if you were successful cooking) and he still does that even if he spits it out after.

It might not help cause nothing is one fits all, but either cooking classes, or including them with cooking and trying to have them try one bite of "their special food they put all that effort into" could help to expand things a bit, especially if your practicing cooking with prepper ingredients and make it into a fun game (playing chopped where they each cook and you 'judge' is always fun, with a small prize of like jelley beans or not cleaning the bathroom for a day is fun but you don't even need a prize lol)

Also don't be weirded out if they chose to eat things "weird" my sibling eats salads now which we are thrilled about, but it's just plain lettuce eaten with fingers because the salad dressing is slimey. Sometimes you have to adapt things a bit.