I love that this was traditionally made using sourdough starter, and wonder if there are any tips for converting (back) to this from commercial ye...

Hi Antonia! My starter is 100% hydration (equal parts water/flour). Thank you for your help!!

Okay, here goes.
Do as it says, except reduce yeast to 1/2 teaspoon, and if you need to proof it (I prefer instant yeast because it doesn't need this extra step) do so in 138 grams of the liquid. Mix 60 grams of starter into the 138 g water + yeast mixture until well blended. Proceed with the recipe except use only 220 grams of flour. If that works well, the next time you can try 80 grams of starter and reduce the liquid and flour each by 40 grams, not 30 (as I've just done). I like to start with a small percentage the first time I convert.
You'll have a much longer rise time, as you know, using less yeast. Let us know how it turns out!
Cheers,
AntoniaJames P.S. If you want to use no yeast at all, expect an even longer rise time, and make sure your starter has been well fed regularly for 3 or 4 days before trying this. I'd use the small bit of yeast here, given the near preponderance of raisins.

Thanks again! This is so helpful. Appreciate you taking the time.

You're welcome. Thinking about this a bit more, it occurred to me that I'd probably use 148 and not 138 grams of liquid, given the low hydration ratio - only 67% -- as originally formulated. Have fun! ;o)

Thanks so much Ted! Will check out.

Makes total sense! Thanks for providing your insight. I'll experiment and update if I'm successful :)

Makes total sense! Thanks for providing your insight. I'll experiment and update if I'm successful :)