Potato and Rutabaga Gratin

Ingredients

• 1 ½ lb. Yukon Gold potatoes, scrubbed and cut into ⅛-inch-thick slices (from 5 potatoes)
• 1 (1-lb.) rutabaga, peeled, trimmed, and cut into ⅛-inch-thick slices
• 4 (5-inch) fresh thyme sprigs, plus leaves for garnish
• 1 ½ tsp. kosher salt
• 1 tsp. grated garlic
• ¾ tsp. Dijon mustard
• ½ tsp. black pepper
• 2 ½ cups whole milk, divided
• 3 Tbsp. all-purpose flour
• 4 oz. Gruyère cheese, shredded (about 1 cup), divided

Instructions

1. 1Preheat the oven to 400°F and position the rack about 10 inches from the heat source.
2. 2Place the sliced potatoes, rutabaga, thyme sprigs, salt, grated garlic, Dijon mustard, black pepper, and 2 cups of the milk into a large saucepan.
3. 3Warm the mixture over medium heat until it reaches a simmer, turning the vegetables gently. The liquid will not cover the vegetables completely. Place a lid on the pan and lower the heat.
4. 4Cook gently for about 12 minutes until the vegetables are mostly tender, stirring and turning them occasionally so they do not stick to the bottom.
5. 5Combine the flour and the remaining half cup of milk in a small bowl, whisking until smooth. Pour this slurry into the simmering vegetables.
6. 6Stir the mixture occasionally and cook for about 1 minute until the liquid thickens. Take the pan off the heat and pull out the thyme sprigs.
7. 7Transfer half of the cooked vegetables and sauce into a 10-inch broiler-safe cast-iron skillet or a 2-quart baking dish. Scatter half a cup of the shredded Gruyère cheese over the top.
8. 8Layer the rest of the vegetable mixture over the cheese, arranging the potato and rutabaga slices evenly. Finish by sprinkling the remaining half cup of cheese over the surface.
9. 9Bake uncovered for 15 to 20 minutes until the edges are bubbling. Turn the oven setting to broil while leaving the dish inside.
10. 10Broil for 1 to 2 minutes until the cheese develops a golden brown crust. Take the gratin out of the oven and let it rest for 10 minutes. Garnish with fresh thyme leaves before serving.

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