1. Heat oil in a large saucepan. Add leeks and shallots and sauté for about 15 minutes, until soft and golden. Add baby bella mushrooms and sauté an additional 10 minutes until soft.
2. Add chicken soup, wine, and thyme, and season with salt and pepper to taste. Stir and bring to a low simmer.
3. Meanwhile, wash and clean chicken breasts and pat dry. Using a sharp knife, slice each piece about three-quarters of the way through, making eight to 10 horizontal slits about half an inch (one centimeter) apart. Sprinkle both sides of the chicken with salt, pepper, and garlic powder.
4. Working with one chicken breast at a time, place a portobello mushroom slice into each slit, pressing them all the way to the bottom of the cut so that they won’t slide out during cooking. Match the size of the mushroom slices to the width of the chicken by placing the wider slices in the center of the chicken and the shorter ones towards the ends.
5. Place chicken gently into the simmering sauce. Cover pan and cook over low heat for 20–25 minutes, or until done. Remove chicken from pan. Cover to keep warm and set aside.
6. Using a strainer, strain the sauce from the pan over a bowl and return the leeks, shallots, and mushrooms to the pan. Be sure to reserve the liquid.
7. Add the flour to the pan and mix into the mushrooms and onions until thickened and flour disappears. Pour one cup of the reserved liquid back into the pan, along with the nondairy creamer. Mix well. Cook for two to three minutes until smooth and creamy.
8. To serve, spoon several tablespoons of sauce onto a plate. Place a piece of chicken in the center and top with freshly chopped parsley.