Step 1

Pour the chicken broth into a saucepan with the onion and garlic; bring to a boil and cook at a boil until the liquid reduces by half, about 10 minutes. Pour in the white wine, return to a boil, and reduce the liquid by half again, about 10 minutes. Stir in mushrooms, rosemary, and sage; simmer until the mushrooms are tender and have given up their juice, about 15 more minutes. Stir in cream, butter, lemon, and capers; reduce heat and simmer the sauce until thickened, about 10 minutes (45 minutes in all).

Step 2

Preheat oven to 350 degrees F (175 degrees C).

Step 3

Place the cornstarch into a shallow bowl and press the chicken thighs into the cornstarch on all sides until thoroughly coated. Shake off excess cornstarch. Heat the olive oil in a skillet over medium heat; pan-fry the chicken thighs until they develop a golden brown crust, about 8 minutes per side. Place the chicken thighs into a 9x9-inch baking dish and pour the sauce over the chicken.

Step 4

Bake in the preheated oven until the sauce is bubbling and the chicken is no longer pink in the center, about 15 minutes. An instant-read thermometer inserted into the center of a chicken thigh should read at least 165 degrees F (74 degrees C).