Rinse chicken breasts and pat them dry with paper toweling. Rub with Cajun spice and set aside.
Preheat oven to 350 degrees F.
Melt butter over medium-high heat in a large skillet or saute pan with an oven safe handle, and saute scallions and mushrooms until they are tender and the mushrooms begin to give up their juices. With a slotted spoon remove mushrooms and scallions to a utility platter and keep covered (with aluminum foil or a lid) in a warm place. (Leave room on the platter for the chicken.) Add seasoned chicken to the skillet, searing all sides. Deglaze the pan with white wine, let sizzle for a few minutes and spoon the wine and juices over the chicken to coat, and then cover pan and transfer to oven. Bake until fork tender, about 25 minutes.
While the chicken is in the oven, heat the mushroom soup in a small saucepan and whisk in demi-glace to give it additional flavor. Cover and let simmer over low heat.
Remove chicken from oven (be sure to use an oven mitt - the handle on that pan will be hot!) and use tongs to transfer the chicken to the utility platter (alongside the mushroom mixture). Cover and keep warm.
Add about 1/2 cup water to the pan in which the chicken was baking and stir to loosen the browned bits of flavor on the bottom. Place a strainer over the pot of mushroom soup and demi-glace and (still using an oven mitt for that hot handle) pour the juice from the chicken pan through the strainer into the sauce. Whisk together the sauce and pan juices and let cook for a few more minutes to integrate flavors.
Place chicken on serving dish. Spoon sauce over. Top with sauteed mushrooms/onions and garnish with chopped parsley.