Coat both sides of the chicken breast in the seasoned flour.
Heat oil and 1/4 tablespoon butter in a frying pan over medium-high heat until the butter has melted and starts to bubble. Cook the chicken, turning down the heat if needed, until both sides are golden-brown and cooked through, about 3 minutes on each side. Transfer the chicken onto a serving plate and keep warm.
Heat the remaining 1/2 tablespoon butter (no need to wash the pan). Add in the onion and garlic and cook until fragrant. Add in the mushrooms and cook until the mushrooms have reduced. Add in the chicken stock and cook until the liquid has reduced by half. Season with salt, black pepper and herbs (if using). Scoop the sauce over the chicken and serve.