
Preheat your oven to 425 degrees F and line a baking sheet with foil.
Heat the butter (1 tablespoon) with the olive oil (1 tablespoon) in a large skillet over medium-high heat. Season both sides of the chicken thighs with kosher salt and black pepper, liberally. Sear the chicken skin-side-down for 3-4 minutes, or until it's browned and releases easily from the pan. Remove the chicken to the prepared baking sheet (skin side up). Bake for about 30 minutes, or until internal temperature of the chicken is 165 degrees F.
While the chicken is baking, make the mushroom sauce. Add the sliced onion and mushrooms (16 oz.) to skillet along with a pinch of salt and pepper and the fresh thyme (1 tablespoon), and sauté over medium-high heat, stirring only occasionally, until mushrooms are browned (5-10 minutes).
Add the flour (2 tablespoons) to the mushroom and onion mixture and stir to coat, continuing to cook for another one minute.
Gradually pour in the chicken broth (1 cup). Stir and continue to cook over medium heat, until sauce has thickened. Stir in the heavy cream (1/2 cup). Turn off heat and taste and adjust seasoning if necessary.
When the chicken is done cooking, remove from oven and place in the skillet with the mushroom sauce, along with a spoonful or two of the juices from baking.
Serve chicken with mushroom sauce on top.