Step 1

In a large skillet, heat 2 tablespoons of the butter over medium heat. Add the onion and cook, stirring occasionally, until softened, 4 to 6 minutes. Add the garlic and cook, stirring occasionally, until fragrant, about 1 minute.

Step 2

Add the remaining 3 tablespoons of the butter to the skillet over medium low heat. Add the flour and cook, stirring constantly, until smooth and very lightly browned, about 1 minute. Gradually whisk in the cream until smooth and well combined. Whisk in the salt, thyme, and pepper. Bring the mixture to a simmer, whisking constantly, over medium heat. Remove from heat and whisk in the Cheddar and Gruyere.

Step 3

Using a mandoline or slicer, slice the potatoes into 1/8-inch slices (very thin).

Step 4

Lightly grease a 6-quart slow cooker with nonstick cooking spray. Layer about 1/3 of the potato slices in the slow cooker and top with 1/3 of the cheese mixture. Repeat the layers twice.

Step 5

Cover and cook on LOW until the potatoes are tender, about 6 to 7 hours. (Insert a paring knife into the center of potatoes. If it goes in and comes out easily, they are finished.) Turn off the slow cooker and uncover. Let the potatoes cool and thicken for 20 minutes before serving. Garnish with black pepper and more thyme.