In the bowl of a stand mixer, whisk together the flour blend, sugar, instant yeast, psyllium husks, and kosher salt. Add the mashed potatoes, butter, and preserved potato water (or tap water).
Using the dough hook, begin kneading the dough and then add the egg and continue kneading for 5 minutes until smooth. Scrape down the sides of the bowl and mound the dough into the middle.
Cover and allow to proof in a warm, draft-free area until doubled in size, which could range anywhere from 1-3 hours, depending on the warmth of your room. *(I FIND THE BEST WAY TO PROOF GF DOUGH IS TO TURN ON THE OVEN OR CLOTHES DRYER FOR A FEW MINUTES, TURN IT OFF, AND PLACE THE DOUGH IN THE WARM ENCLOSED AREA).
Once dough has fully proofed, refrigerate it at least 6 hours, preferably overnight. Refrigeration is ESSENTIAL to getting the proper texture for kneading and shaping the dough. If your dough is very sticky, you may need to refrigerate it longer.
The next day, or up to 3 days later, remove the dough from the refrigerator and place it on a well-floured surface (using extra Kim's gf bread flour blend for bench flour). Flatten it slightly and add 3 oz (85g) of the shredded cheddar cheese to the top. Knead the cheese into the dough (I find the cheese flavor remains noticeable in the final bread when it's kneaded in later versus adding it when first making the dough).
Shape dough into a round "boule" and either place it on a parchment-lined baking sheet or set it in a bread banneton. Cover loosely and let rise in a warm, draft-free area until doubled in size (again, this could take shorter or longer depending on the warmth of your proofing area), but should take around 1-2 hours if your proofing area is warm enough.
Place Dutch oven in oven and preheat to 225° C. (See notes for alternative if you don't have a Dutch oven).
When dough has fully risen, place it on a piece of parchment and brush it with milk. Carefully coat the dough with the remaining shredded cheddar cheese (1.5oz or 43g) and carefully lower it into the hot Dutch oven. Slash the top with a bread lame or sharp knife and replace the cover.
Bake bread for 45-50 minutes, or until the bread is deep golden brown with a crunchy crust. If you tap the bottom, it will sound hollow. Allow to cool completely before cutting. Internal temperature 88° C