Carrot and Sharp Cheddar Gratin

This gratin is made a little differently from the others—the carrots are parcooked in the cream, adding an extra infusion of flavor.

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  1 oz. (2 Tbs.) unsalted butter; more at room temperature for the dish
  
  


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  1-1/2 Tbs. extra-virgin olive oil
  
  


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  1 medium yellow onion, cut into 1/2-in. dice (about 1-1/2 cups)
  
  


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  Kosher salt
  
  


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  1 cup plain panko
  
  


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  2 Tbs. chopped fresh flat-leaf parsley
  
  


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  2 tsp. chopped fresh thyme
  
  


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  1 cup heavy cream
  
  


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  1 Tbs. Dijon mustard
  
  


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  Freshly ground black pepper
  
  


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  3 lb. large carrots (about 12), peeled, halved lengthwise, and cut crosswise into 1/2-inch-thick half-moons
  
  


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  4 oz. coarsely grated sharp Cheddar (1 cup)
  
  

Position a rack in the center of the oven and heat the oven to 350°F. Butter a 9x13-in. (or similar) baking dish. In a 12-in. skillet, heat the oil over medium-high heat. Add the onion and 1/2 tsp. salt and cook, stirring occasionally with a wooden spatula, until golden-brown, 7 to 10 minutes.

Meanwhile, melt the butter and pour into a medium bowl. Add the panko, parsley, and thyme and toss well.

Whisk the heavy cream, mustard, 1-1/2 tsp. salt, and 1/4  tsp. pepper into the onion, scraping up any browned bits, and then stir in the carrots. Bring just to a boil, cover, lower the heat to medium low, and simmer until the carrots are crisp-tender, 10 to 12 minutes.

Spread the carrot mixture evenly in the prepared dish. Scatter the Cheddar over the carrots, top with the panko mixture, and bake until the carrots are tender when pierced with a fork and the crumbs are golden-brown, 30 to 40 minutes.

Let rest for 15 minutes before serving.