• 5 Tbsp unsalted butter, divided
• 1 small yellow onion, chopped
• 3 garlic cloves, chopped
• 1 1/2 Tbsp all-purpose flour
• 2 1/2 C heavy cream (milk will work as well)
• 4 tsp Kosher salt
• 2 tsp thyme, plus more for garnish
• 1 1/2 tsp black pepper
• 1 1/2 C sharp Cheddar cheese, shredded
• 4 1/2 C Provolone cheese, shredded
• 4 large potatoes, peeled and sliced
Melt 2 Tbsp of the butter over medium heat in a large skillet. Add onion and cook, stirring occasionally, until softened. Add garlic and cook, stirring occasionally, until fragrant, about 1 minute. Add the remaining butter. 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 cheeses. Lightly grease a 6-quart slow cooker with nonstick cooking spray. Layer about a third of the potato slices in the slow cooker and top with one-third of the cheese mixture. Repeat the layers twice. Cover and cook on low until the potatoes are tender, about 6 to 7 hours. Turn off the slow cooker and uncover. Let the potatoes cool and thicken for 20 minutes before serving.
