I think utlimately it really doesn't matter if it's the sound or smell that is making them scared.
Make whatever it is (sound or smell, but ultimately it's you cooking) become the predictor of good stuff. Like treats or 'out' time. Think Pavlov's dogs.
Turn the oven on, start cooking and let your bunny out with some treats.
Make out time only happen when the oven is off.
This will then become a CS for out time/treat time.
Have patience and keep at it-it's called counter conditioning (making something that means x mean y instead)
It's super easy and if you give treats or out time every time you cook you'll see a change quickly I'm sure!