Preheat your oven to 375°F (190°C). Butter a 9×13-inch baking dish generously.

Slice the potatoes about 1/8 inch thick using a mandoline or sharp knife. Rinse the slices in cold water and pat dry with a clean towel to remove excess starch.

In a medium saucepan over medium heat, melt the butter. Add minced garlic and cook for about 1 minute until fragrant. Whisk in the flour and cook for another minute.

Slowly whisk in the heavy cream, stirring constantly until it thickens slightly (about 3-4 minutes). Remove from heat and stir in 1½ cups of shredded Gruyere, fresh thyme, salt, and pepper.

Arrange half the potato slices in an even layer in the baking dish. Pour half the cheese sauce evenly over the potatoes. Repeat with remaining potatoes and sauce. Sprinkle the remaining ½ cup Gruyere on top.

Cover the dish tightly with foil and bake for 40 minutes. Remove foil and bake for an additional 20-30 minutes until the top is bubbly and golden brown and potatoes are tender.

Let the scalloped potatoes rest for at least 10 minutes before slicing and serving.