Preheat your oven to 200C (390F) and grab a 9 x 13" baking dish (this dish is a little deep however I find it saves the white sauce spilling over into the bottom of the oven).
Grate the potatoes using a box grater (I have chosen to leave the skin on but you can peel yours if you prefer). Place the grated potato onto a flour sack tea towel and squeeze out as much liquid as you can. Set the potatoes aside in the dish towel while you prepare the other ingredients.
In a medium sized saucepan, melt the butter gently. Once the butter is melted, add the sourdough starter and whisk until combined. Then add the milk on top and whisk over a medium heat until the sauce thickens. Add the pepper, salt and onion powder and whisk until well combined. Depending on which type of pepper you use, the sauce might look a grey or brown color - don't stress, it will be fine!
Now take the saucepan off the heat and whisk the sour cream through the white sauce you've just made. Set this aside for now.
In a large bowl, add the potatoes and about ¾ of the cheese and stir together until well distributed. Now pour the white sauce into the bowl and stir through the potato and cheese mixture.
Pour the potato mixture into your baking dish. Top with the remaining cheese and breadcrumbs (if using). Cover with foil and place into the oven for 45 minutes.
Once 45 minutes has passed, remove the aluminium foil and place back into the oven to brown for 15 minutes (increase the temperature to 220C (430F) for this bit to ensure the top browns).
Once golden brown and bubbly on top, remove from the oven and allow to sit for 15 minutes before serving. This allows the sauce to thicken and the casserole to "set" before you serve it.