Prepare the potatoes by peeling them and thinly slicing them with a mandoline into ⅛ thick slices.
Bring a pot of water to a boil, add a generous amount of salt, about 2 tablespoons. I use Diamond crystal kosher salt.
Add the potato slices and boil them for 2 minutes. Remove them to a tea towel lined baking sheet to remove excess water.
Preheat the oven to 350°F with a rack in the center.
In a medium sauce pan, over medium-high heat add the butter and shallots and cook for 2 minutes.
Add the minced garlic and cook for 20 seconds until aromatic.
Add the flour and garlic powder and mix well until no flour clumps remain.
Add the white wine or broth and cook for 1 minute until a paste forms.
Add the milk and bring to a gentle boil.
Add the thyme and cook for another 2-3 minutes.
Prepare a casserole dish. Dip the potato slices into the creamy sauce using kitchen tongs and arrange them in the casserole dish in a single layer with the potatoes standing vertically. I used a round baking dish so I layered them around the dish into a rosette pattern.
Cover the dish with aluminum foil and bake for 30 minutes.
Bake uncovered for another 20. Check the potato consistency and bake for additional 10 if needed.
Allow the dish to rest for 10-15 minutes before serving.