Clean mushrooms and slice in thick slices. Heat a little of the olive oil in heavy frying pan and saute mushrooms 4-5 minutes, until starting to slightly brown.
While mushrooms brown,then pound chicken until it is about 3/8 inch thick. (Put chicken in a small, heavy duty ziploc bag and use a rolling pin or meat mallet to pound one chicken breast at a time.)
Mix flour and poultry seasoning on a plate. Remove mushrooms from frying pan and put in small bowl. Add a bit more olive oil to pan and heat. Coat each chicken breast in a very thin layer of flour/poultry seasoning mixture, then add to pan. Saute over medium heat until quite brown on each side, about 6-7 minutes total. While chicken cooks, puree the garlic combine balsamic vinegar, Dijon, and garlic puree.
When chicken is browned on both sides and done, remove from pan and put on serving plate. Add chicken stock to hot pan and deglaze, scraping browned bits off the bottom. Cook 1-2 minutes, until stock is slightly reduced. Add balsamic vinegar mixture, and mushrooms, cook 1-2 minutes more. Pour over chicken and serve immediately.