Trim any fat off the chicken. Season on both sides with 2 teaspoons of salt.
Heat the olive oil in a large, deep skillet or saute pan over medium heat.
Add the chicken breast arrange them so that they do not overlap.
Cook them for 5 minutes on each side to brown. The chicken should release and be easy to turn once a crust forms on the bottom. Give it another minute or two if it is sticking to the pan.
Take the pan off the heat and remove the chicken from the pan to a plate. Don’t worry that it is not cooked through. It will finish cooking in the sauce. Loosely cover with foil to keep warm.
Do not wash or wipe the pan. Place it back over medium heat and add the butter.
When the butter is melted and foamy add the onions and mushrooms. Cook them gently for 3-4 minutes until the onions soften, the mushrooms are golden in spots, and any liquid rendered has cooked out.
Add the flour to the onion mushroom mixture. Cook for 1 minute, stir frequently.
Slowly add the chicken broth while stirring briskly with a whisk or wooden spoon until the flour dissolves. Gently scrape any bits off the bottom of the pan.
Add the mushroom soup, garlic powder, Italian seasoning, and black pepper. Stir until incorporated.
Add the chicken back to the pan along with any juices collected on the plate. If the sauce does not come to a simmer right away raise the heat a bit.
Then, lower the heat to medium-low, cover and cook for 12-15 minutes or until the chicken is cooked through (see Recipe notes 1). Flip the chicken once or twice during the cooking time and stir the sauce. Keep the mushroom sauce at a simmer. Raise or lower the heat as needed.
The gravy will thicken further as it cools off. If necessary, take the skillet off the heat, uncover, and let it cool for about 5 minutes to let it set and thicken a bit.