Adjust your oven rack to the lowest position and preheat to 425 degrees.
Place sliced potatoes in a large bowl and fill with cold water. Use hands to mix potatoes and agitate water to rid potatoes of excess starch. Transfer potatoes to a colander to drain. Spread drained potatoes evenly on a large kitchen towel and pat dry. Set aside and allow to dry completely.
Meanwhile: Whisk 7 of the tablespoons of butter, cornstarch, garlic, salt, pepper and rosemary together in a large sized mixing bowl. Add potatoes and gently toss until evenly coated.
Add remaining 1 tablespoon of butter in a large cast iron skillet (you can use a heavy bottomed 10’’ nonstick skillet or a spring form pan, but you will not achieve same golden color). Swirl butter to evenly coat the bottom of the skillet.
Starting in the middle of your skillet, place one slice of potato. Working in a circular motion, start adding potato slices, one at a time, slightly overlapping the previously placed potato. Once you have a full inner circle formed, sprinkle with some of the cheese. Then move outward forming another complete circle and sprinkling with cheese. Keep placing and overlapping potato slices, trying to keep a level, even thickness. One you have covered the bottom of the skillet, place the remaining potatoes and cheese evenly over the circles of potatoes you have formed, again keeping the potatoes level.
Heat the cast iron skillet over medium high heat and cook 5 minutes. Spray the top of the potatoes with nonstick cooking spray. Place a large piece of foil on top of the potatoes. Add a heavy skillet, or cake pan, on top of that to weigh down potatoes and compress them. Fill the skillet with dry beans, weights, bricks or rocks.
Transfer the skillet to the oven and bake for 25 minutes.
Remove the top skillet with weights and the foil from the top of the potatoes. Continue to cook the galette an additional 15-20 minutes, or until the tops of the potatoes are golden brown in color, cooked through and tender.
Transfer cast iron skillet to a stove top burner, over medium heat. Season with salt and pepper. Cook 2-3 minutes and gently shake the skillet back and forth, until you see the galette releasing itself from the sides of the pan.
Remove from heat and allow to cool slightly for 5-10 minutes. Run a paring knife around the outside of the galette to help it loosen. Place a cutting board, large clean flat platter over the skillet. CAREFULLY flip the skillet over to invert the potatoes onto the board. Gently lift the skillet off of the potatoes.
Sprinkle more cheese on top (optional) and tent the potatoes lightly with foil and allow to rest 3-5 minutes before carefully cutting it into wedges. Serve with optional garnishes.