Wash and quarter the red potatoes into evenly sized pieces, approximately 1 inch across.
Toss with olive oil, rosemary, salt, and black pepper. Arrange in a single layer on a large, rimmed baking sheet, leaving space between the potatoes.
Roast for 30–35 minutes, turning halfway through, until golden brown on the edges and tender in the center.
Start the potatoes before preparing the chicken so the two dishes can finish around the same time.
Season the chicken
Pat the chicken thighs completely dry with paper towels.
In a small bowl, combine the sweet paprika, smoked paprika, dried rosemary, garlic powder, onion powder, crushed fennel seeds, salt, and black pepper.
Rub the seasoning evenly over both sides of the chicken thighs.
Sear the chicken thighs
Heat 1 tablespoon olive oil in a large, oven safe cast iron skillet over medium heat.
Place chicken thighs skin side down and sear for approximately 7–10 minutes, without moving them, until the skin is deeply golden and releases easily from the pan.
Flip and sear the opposite side for another 2 minutes.
Transfer the chicken to a plate. It will finish cooking in the oven.
Cook the apples and onions
Reduce the heat to medium low and carefully remove excess fat from the skillet, leaving about 1 tablespoon.
Add the sliced onions and cook for 5–7 minutes, stirring occasionally, until softened and beginning to caramelize.
Add the apple wedges and cook for another 2–3 minutes.
Stir in the minced garlic and cook for approximately 30 seconds, until fragrant.
Make the apple cider Dijon sauce
Pour the fresh apple cider and chicken broth into the skillet, scraping the bottom with a wooden spoon to release the browned bits.
Whisk in the Dijon mustard until incorporated.
Allow the sauce to simmer gently for 2–3 minutes, developing a slightly richer consistency.
Finish the chicken in the oven
Return the chicken thighs to the skillet, skin-side up, nestling them among the apples and onions.
Keep the crispy skin above the liquid to prevent it from becoming soggy.
Transfer the uncovered skillet to the 425°F oven.
Roast for approximately 20–30 minutes, or until the thickest part of the chicken reaches at least 165°F (74°C), without touching the bone. Chicken thighs often become more tender around 175–185°F.
Rest and serve
Remove the skillet from the oven and allow the chicken to rest for 5–10 minutes.
If you prefer a thicker sauce, temporarily remove the chicken and simmer the pan juices on the stovetop for a few minutes until reduced.
Taste and adjust the salt and pepper.
Serve the chicken with the rosemary roasted potatoes and spoon the warm apple cider Dijon sauce over the chicken. Garnish with fresh rosemary or sage.