Preheat oven to 400 degree F.
Spray an 8 x 11 inch casserole dish with non-stick spray and dot the bottom with butter. Place it in the oven until butter melts then remove the pan from the oven.
In a medium saucepan, heat heavy cream, Boursin cheese, salt and pepper over medium-low heat just until the cheese melts and stir until mixture is smooth. Set mixture aside until ready to use.
Slice the partially cooked potatoes with a mandolin (if available) into 3/16” slices. Place the sliced potatoes in a large bowl and pour the cheese/cream mixture over all them.
(This next step is messy, but I don’t know any other way to do it.)
Pick up several potato slices and place them in the casserole dish. Organize the slices into a small stack then place them vertically along the side of the dish. Repeat with all the potato slices, placing them where needed. The potatoes should be tightly packed.
Pour the excess cheese/cream mixture over the potatoes until the mixture comes up almost to the top.
Cover the dish with foil and place it on a foil-lined baking sheet pan. Place the dish in the preheated oven and bake 30 minutes. Remove the foil covering from the dish and bake another 30 minutes then sprinkle grated Parmesan over the potatoes. Bake 15-30 more minutes until the top is golden brown, the middle is bubbly and potatoes are cooked through. Total cook time is 75-90 minutes.
Remove potatoes from the oven, all to rest 5-10 minutes before serving.
**We have made these potatoes both the “Hasselback” way and the traditional way (layering). Both variations are delicious. The potatoes look prettier made the Hasselback way. The traditional way is less messy, easier and, in my opinion, creamier.
If you want to make them the traditional way, cook them in a 9x13 inch casserole dish, use 3 lbs. of potatoes and 1 package of Boursin cheese. Layer half the potatoes then half the cheese/cream mixture. Repeat. Follow the same cooking instructions. The potatoes will probably cook more quickly – 45-70 minutes instead of 90. Cook at 350 degrees F.