- 1
Set an oven rack to the middle position and preheat the oven to 400°F. Grease a 9x13-inch baking dish (shallow 3-qt.) with 1 tsp. butter. Scatter half of the shallots on the bottom of the pan, and shingle ⅓ of the potatoes on top; set aside.
- 2
Melt the remaining 3 Tbsp. butter in a large saucepan over medium heat. Add the garlic and thyme and cook about 1 minute until fragrant. Add the flour, salt and pepper; cook about 1 minute, whisking constantly, until combined and fragrant.
- 3
Gradually whisk in the stock until smooth, then whisk in the milk; bring to a simmer over medium-high heat. Reduce heat to low. Stir in 2 cups of the cheese until melted and smooth.
- 4
Pour ⅓ of cheese sauce over the potato layer. Scatter remaining shallots on top. Repeat layers of potatoes and sauce twice, then sprinkle with remaining ½ cup cheese.
- 5
Cover the pan with foil and bake 50 minutes. Uncover; continue to bake about 20 minutes longer until sauce is bubbling, cheese on top is golden brown and potatoes are tender when pierced with a paring knife. Let stand 10 to 15 minutes; sprinkle with chives before serving.