Sprinkle the chicken with black pepper and salt, and place in the pan. Brown the chicken with skin on, let it cook for about 5 minutes then flip to the other side and let it brown. Transfer to a plate and set aside.
Leaving the oil in the pan, add the onions and sauté, then add the garlic. Sauté until garlic is aromatic and make sure to keep stirring as it can get burnt easily.
Add the red wine, bullion and a bay leaf to the pan. Return the chicken to the pan, and make sure that the chicken is well coated with the sauce. Cover with lid and simmer for about 40 minutes on medium low heat.
When chicken is cooked and tender, transfer it to a plate. Mash the butter with the flour, then add the paste along with the mushrooms to the sauce. Let it simmer on medium low heat for 10 minutes, the sauce should thicken and reduce.
Return chicken to the sauce to reheat it and coat with the sauce for a few minutes.