Cut both chicken breasts in half lengthwise, creating 4 thinner pieces. Season with salt (1 teaspoon) and pepper (1/2 teaspoon).
Heat olive oil (2 tablespoons) to a 12-inch skillet over medium heat. When the oil shimmers add chicken breasts then cook each side for 4-5 minutes. Transfer chicken to a plate.
Melt butter (2 tablespoons) in the skillet. Add onions (1/2 cup) and mushrooms (8 ounces) and sauté over medium-high heat, stirring often, until the onion is translucent and the mushrooms are soft and have released their liquid, about 7-9 minutes. Stir in minced garlic and continue to cook, stirring often, until garlic becomes fragrant, about 1 minute.
Stir in condensed cream of mushroom soup (1 can), chicken broth (3/4 cup), Better Than Bouillon (1 teaspoon), sherry (1 tablespoon), and thyme (1/2 teaspoon). Stir until soup and Better Than Bouillon is dissolved. Increase heat and bring to a boil. Immediately reduce heat and simmer.
Return the chicken, along with the juices left on the plate, to the skillet and nestle it into mushrooms and sauce. Simmer for 5-6 minutes or until the internal temperature of the chicken reaches 165ºF (74ºC).
Notes
Careful With Cooking Time: This recipe is simple and fast. Don't overcook the chicken, as you will add it back to the pan for a quick simmer.
Slice In Half: To slice the chicken breast, use a sharp chef’s knife. Firmly place your hand on the breast and place your knife at 90 degrees at the thicker side of the breast. You may need to rotate the breast so that the thicker part is facing your knife. Keeping your knife level, slice across the width of the breast approximately halfway between your hand and the cutting board.