Carefully, cut each chicken breast in half horizontally to get 2 thinner pieces per breast. Season with salt and pepper. Set aside.
In a skillet or non-stick pan, add olive oil and chicken breasts without overlapping each piece. Sear on medium-high heat and flip chicken and sear the other side once pan-side is golden and caramelised. Remove chicken from pan and set aside.
In the same pan, add butter, sliced mushrooms, salt and pepper and saute on high heat until golden brown and nutty. Only turn mushrooms every minute or 45 seconds or when pan side is golden brown. This will take about 4 minutes. (note 1)
Once mushrooms are brown and reduced in size by about half, add garlic and spinach. Continue to saute on high heat until spinach is wilted. (note 2)
Pour in chicken stock, deglazing the pan, allow the stock to reduce by half.
Reduce the heat to low and pour in heavy cream. Bring it to simmer then add chicken breasts back in the pan. Simmer for 1 minute or until chicken is cooked. Taste the sauce, adjust for salt and pepper if required, to taste.