Begin by seasoning your chicken breasts with a pinch of salt and pepper. Heat a tablespoon of oil in a large skillet over medium-high heat until shimmering. Sear the chicken for 3-4 minutes on each side until golden brown, then transfer to the slow cooker.
In the same skillet, add sliced mushrooms and cook for about 5 minutes until they release their moisture and turn a deep golden color. Add the sliced onion and minced garlic, cooking for another minute until fragrant and slightly translucent. Transfer this mixture to the slow cooker.
Pour the Marsala wine into the skillet, scraping up any browned bits from the bottom with a wooden spoon. Bring it to a gentle simmer and cook for about 2 minutes to concentrate the flavor. Pour this wine mixture into the slow cooker over the chicken and vegetables.
Add the chicken broth to the slow cooker, ensuring the chicken and vegetables are mostly submerged. Cover with the lid and set the cooker to low. Let it cook for 4 to 6 hours until the chicken is tender and the flavors meld beautifully.
Once cooking time is up, carefully remove the chicken breasts and set aside. Stir a tablespoon of butter into the sauce in the slow cooker to add shine and richness, then return the chicken to the sauce and cook uncovered for an additional 10 minutes to absorb the flavors.
Finally, plate the chicken and generously spoon the mushroom and wine sauce over each piece. Let it rest for a few minutes before serving. Enjoy this tender, wine-infused dish with your favorite pasta or crusty bread to soak up the luscious sauce.
