Season your chicken thighs with some sea salt and pepper. In a shallow dish, add the flour and dredge your chicken thighs firmly on both sides, shaking off the excess flour; set aside.
In a small saucepan add your apple cider, rosemary sprig, mustard seeds, Dijon mustard, and garlic; whisk well to combine. Place the saucepan over medium-low heat and bring to a low boil. Simmer until liquid is reduced by half, about 10 minutes.
Once your sauce is done, remove the rosemary sprigs and discard them.
Heat a heavy-bottomed skillet over medium heat, add in the oil, and heat until sizzling.
Once hot carefully add the chicken thighs and sear each side until nicely golden brown about 4-5 minutes on each side.
Set chicken aside on a plate.
In that same preheated skillet, add in your diced onions and cook until just softened.
Pour in your apple cider sauce and using a wooden spoon gently stir to incorporate the brown bits from the bottom of the pan.
Bring this sauce to a boil, then reduce the heat to a medium-low.
Return your browned chicken thighs back to the skillet and simmer for 8-10 minutes, or until chicken is cooked through and the sauce is nicely thickened.
Enjoy!