Pat the chicken thighs dry and season both sides with the salt, pepper, and garlic powder.
Heat the olive oil in a large skillet over medium-high heat. Add the chicken thighs and cook for 5–6 minutes per side, until golden brown. Transfer to a plate.
Reduce the heat to medium and melt the butter in the same skillet. Add the sliced apples and cook for 3–4 minutes, stirring occasionally, until they begin to soften. Stir in the garlic and cook for 30 seconds.
Stir in the apple cider, Dijon mustard, and thyme, scraping up any browned bits from the bottom of the skillet.
Return the chicken thighs and any accumulated juices to the skillet. Cover and simmer for 8–10 minutes, or until the chicken reaches an internal temperature of 165°F and the apple cider has reduced into a light sauce.
Spoon the apples and sauce over the chicken and serve warm with mashed potatoes, roasted sweet potatoes, rice, or roasted vegetables.
Pat the chicken dry: Remove excess moisture with paper towels before seasoning to help the chicken develop a golden-brown crust.
Don't overcrowd the skillet: Give the chicken thighs enough room to brown evenly. Cook in batches if necessary.
Slice the apples evenly: Cut the apples into similar-sized slices so they soften at the same rate without becoming mushy.
Scrape up the browned bits: Stir the apple cider into the skillet and scrape the bottom to incorporate the flavorful bits left from searing the chicken.
Use a meat thermometer: Chicken thighs should reach an internal temperature of 165°F before serving.