Preheat oven to 425°F (220°C). Prepare 2 sprigs of rosemary finely chopped. Toss apple slices and cubed sweet potatoes in a bowl with 1 tablespoon of olive oil and chopped rosemary. Set aside.
Pat dry chicken thighs with paper towels and season with salt and ground black pepper. Heat a skillet with olive oil over medium-high heat. Arrange chicken thighs in the skillet with skin side down, and sear for 3 to 4 minutes on each side until golden brown and halfway cooked. Transfer to a plate.
In the same skillet, melt the butter and whisk in a mixture of apple cider, honey and Djion mustard. Bring to a simmer, add chicken thighs, apple slice, sweet potatoes and 2 sprigs of rosemary.
Turn off the heat, and bake the entire dish in the oven for 15 to 20 minutes or until the chicken has cooked through, and the sweet potatoes are tender.