Sheet-Pan Chicken Thighs with Potatoes, Fennel, and Mustard-Beer Sauce

Ingredients

• 1 1/2 pounds Yukon gold potatoes, sliced crosswise into 1/8-inch-thick rounds
• 1 large bulb fennel, stalks, and root end trimmed and bulb sliced lengthwise 1/8-inch-thick, fronds reserved
• 1 tablespoon extra-virgin olive oil
• Kosher salt and freshly ground black pepper
• 8 bone-in, skin-on chicken thighs
• 4 ounces sharp white Cheddar cheese, finely grated, preferably with a Microplane grater
• One can lager, pilsner, or pale ale
• 3 tablespoons stone-ground mustard
• 1 teaspoon sugar, or more to taste
• 2 tablespoons unsalted butter

Directions

1. Heat the oven to 450 degrees F (230 degrees C), with a rack placed in the upper-middle position. Add the potatoes and fennel to a large mixing bowl, drizzle with the olive oil, and toss to coat. Add a light sprinkle of salt and pepper. Spread the potatoes and fennel across a rimmed baking sheet in even, slightly overlapped layers so the pan is fully covered. A neat scalloped look is lovely if you feel like fussing, but not remotely required.
2. Sprinkle the chicken with salt and pepper on all sides. Place the thighs around the outside of the sheet pan, with one positioned over each corner. Those spots run hottest, and the chicken helps protect the vegetables underneath from overbrowning.
3. Roast the chicken and vegetables until the skin is crisp and deeply golden, about 45 minutes. Move the chicken thighs to a warm serving platter or individual plates, and keep them warm.
4. Sprinkle the grated cheese evenly over the potatoes and fennel. Switch the oven to broiler mode. Broil until the cheese has melted and is lightly browned in places, and the top of the vegetables looks a bit crisper and more golden, about 5 minutes.
5. With a slotted spatula, gently lift the potatoes and fennel onto the serving platter or plates, letting extra fat drip off before each transfer. If you want a more gratin-like pile, stack the vegetables as you move them. Keep warm.
6. Place the baking sheet over a stovetop burner set to medium-high heat. Cook the juices left on the pan, shifting the sheet as needed, until they brown on the bottom. Watch your potholder or towel carefully as you move the hot pan over the flame. If the baking sheet warps a bit, don’t panic; that can happen with sharp temperature changes, and it should settle back afterward.
7. Pour the extra fat from the baking sheet into a small heatproof bowl. Put the baking sheet back over the heat and add the beer right away. Raise the heat to high and bring the beer to a boil. Use a wooden spoon to loosen the browned bits stuck to the bottom of the pan.
8. Add the mustard and stir it in with the wooden spoon. Carefully shake the baking sheet while you work the mustard into the beer so the shallow liquid mixes evenly. Keep cooking, stirring and gently shaking the pan, until the sauce has reduced by about half. Stir in the sugar. Turn off the heat, add the butter, and stir and shake constantly until the butter melts into a smooth sauce. Taste, then adjust with salt and/or more sugar, as desired. Transfer the sauce to a serving boat. Scatter the reserved fennel fronds over the potatoes and serve.