gluten freeChop the herbs
If you're using the herbs, rinse and pat dry the tarragon, the parsley, or a mix of both, then finely chop them and set them aside for garnish.
Season the chicken
Pat the chicken thigh fillets dry with paper towel, then season both sides with 0.5 tsp salt and 0.2 tsp pepper.
Sear the chicken
Set a large skillet over medium-high heat and add the olive oil. When it shimmers, lay in the chicken in a single layer and sear until the underside is deep golden, about 5 minutes. Flip and cook for 4 to 5 minutes more, until cooked through, then transfer the chicken to a plate.
Make the sauce
Lower the heat to medium-low. Stir the heavy cream, Dijon mustard, whole grain mustard, the remaining 0.2 tsp salt and 0.2 tsp pepper, and 1.5 tbsp water into the skillet, scraping up the browned bits from the bottom. Let the sauce bubble gently for 2 minutes, stirring often, until it looks glossy and lightly thickened. Return the chicken and any juices to the pan and turn the pieces in the sauce until coated; warm everything through for 1 minute over low heat.
Finish and serve
Spoon the chicken and sauce onto plates or serve it straight from the skillet. Finish with the chopped herbs, if you're using them, and serve hot.
