01 -
Preheat the oven to 375°F and position a rack in the top third of the oven. Cut the chicken breasts in half lengthwise to create six thinner pieces. Season both sides generously with salt and pepper. Dredge the chicken in the 1/3 cup of flour, shaking off any excess.
02 -
Heat the olive oil and 1 tablespoon of butter in a large skillet over medium-high heat. Sear the chicken in batches, 3–4 minutes per side, until lightly browned but not cooked through. Transfer to a 9×13-inch casserole dish.
03 -
In the same skillet, melt 1 tablespoon of butter over medium heat. Add the mushrooms and cook, stirring occasionally, until they're golden and their liquid has evaporated, about 5 minutes. Transfer to the casserole dish, arranging them around and on top of the chicken.
04 -
Return the skillet to medium heat and melt the remaining 2 tablespoons of butter. Add the minced garlic and cook for 30 seconds until fragrant. Stir in the 2 tablespoons of flour and cook for 1 minute, stirring constantly.
05 -
Slowly pour in the sherry, whisking to deglaze the pan, and scrape up any browned bits. Gradually whisk in the heavy cream until smooth. Stir in the optional red pepper flakes if using. Simmer for 2–3 minutes until thickened.