Brussels sprouts with toasted hazelnuts & apple cider-chile butter sauce

Brussels sprouts, trimmed; halved or quartered, if large	1 pound
Olive oil, extra-virgin	3 tablespoons
Rosemary, freshly chopped	½ teaspoon
Salt, kosher
Pepper, freshly ground
Hazelnuts, coarsely chopped	¼ cup
Apple cider, unfiltered	1 cup
Shallot, minced	1 tablespoon
Espelette pepper, Aleppo chile, cayenne pepper, or other chile powder	Big pinch +
Butter, unsalted, very cold	2 tablespoons

1. Preheat the oven to 400°F; line a large rimmed baking sheet with foil. 
2. In a large bowl, toss the sprouts with oil and rosemary; season with salt and pepper. 
3. Spread in a single layer on the baking sheet; roast until tender, about 15 to 18 minutes. 
4. During the last 5 minutes or so of roasting, spread the nuts on a small baking sheet, and slide into the oven; cook until lightly toasted.
5. In a small saucepan, combine the cider, shallot, and pepper; bring to a simmer over medium-high heat. 
6. Cook, stirring and scrapping the sides of the pan occasionally with a spatula, until the liquid is reduced to about 2 tablespoons, about 15 to 20 minutes. 
7. Add the butter, stir until most of the butter is incorporated, the remove the pan from heat as you work in the last of the butter. 
8. Whisk in ¼ teaspoon salt, then adjust seasoning with salt and pepper. 
9. Mound the sprouts on a platter, and drizzle the sauce over the top. 
10. Sprinkle the nuts over the top, and serve immediately with any remaining sauce. 

Recipe adapted from Modern Sauces by Martha Holmberg