Whisk together maple syrup, mustard, cayenne pepper, vinegar, ancho chile powder and ½ tsp. salt in a small bowl. Set aside half of the maple mixture for serving.

Place chicken thighs in a large resealable plastic bag. Pour remaining maple mixture over chicken; seal bag. Turn bag to evenly coat chicken with maple mixture. Place flat in the refrigerator. Marinate for 1 to 2 hours, turning bag occasionally.

Preheat oven to 425°. Place a large rimmed baking pan in oven; preheat pan for 10 to 15 minutes. Cut squash slices into 1-in. pieces; set aside.

Combine 1 Tbsp. oil, remaining ½ tsp. salt and black pepper in a large bowl. Add acorn squash, Brussel sprouts and shallots; toss to coat.

Remove baking pan from oven; brush with remaining 1 Tbsp. oil. Spread vegetables, in a single layer, on half of the prepared baking pan. Drain chicken; discard marinade. Place chicken, boned sides up, on the other half of the pan. Roast for 15 minutes; turn chicken over.

Sprinkle vegetable mixture with slivered almonds; toss. Roast for 16 to 18 minutes more or until chicken reaches 165°. Serve with reserved maple mixture.