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Creamy Potato and Rutabaga Gratin
Bubbling, golden, and impossibly creamy, this gratin gives the classic potato side dish a welcome twist. Earthy rutabaga brings a subtle sweetness that perfectly balances the savory garlic, Dijon mustard, and fresh thyme. You'll simmer the thinly sliced vegetables right in the seasoned milk before layering them with nutty Gruyère cheese. It's a comforting, deeply flavorful bake that easily steals the spotlight at any holiday table or Sunday dinner.
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
- 1Preheat the oven to 400°F and position the rack about 10 inches from the heat source.
- 2Combine the sliced potatoes, rutabaga, thyme sprigs, salt, grated garlic, Dijon mustard, black pepper, and 2 cups of the milk in a large saucepan. Bring to a simmer over medium heat, turning the vegetables gently. The liquid won't cover the vegetables completely.
- 3Cover the pan and reduce the heat to low. Simmer for about 12 minutes until the vegetables are mostly tender. Stir and turn them occasionally so they don't stick to the bottom.
- 4Whisk the flour and the remaining 1/2 cup of milk in a small bowl until smooth. Pour this slurry into the simmering vegetables. Cook for about 1 minute, stirring occasionally, until the liquid thickens. Remove the pan from the heat and discard the thyme sprigs.
- 5Transfer half of the vegetable mixture into a 10-inch broiler-safe cast-iron skillet or a 2-quart baking dish. Scatter 1/2 cup of the shredded Gruyère cheese over the top.
- 6Layer the remaining vegetable mixture over the cheese, arranging the potato and rutabaga slices evenly. Sprinkle the remaining 1/2 cup of cheese over the surface.
- 7Bake uncovered for 15 to 20 minutes until the edges are bubbling. Leave the dish inside and increase the oven temperature to broil.
- 8Broil for 1 to 2 minutes until the cheese develops a golden brown crust. Watch closely so the cheese doesn't burn. Remove the gratin from the oven and let it rest for 10 minutes. The resting time allows the creamy sauce to set up properly before serving.
Notes
- Make ahead: You can slice the potatoes and rutabaga up to a day in advance. Keep them submerged in water in the fridge so they don't oxidize.
- Equipment: A 10-inch cast-iron skillet works beautifully here, as it transitions seamlessly from stovetop to oven to broiler.
- Swap: If you can't find Gruyère, a good quality sharp white cheddar or Comté makes an excellent substitute.
- Reheat: Warm leftovers gently in a 325°F oven until heated through, or microwave individual portions.