Cut the chicken into bite-sized pieces and season with salt and pepper. If you’d like a crispier bite, coat lightly in flour. Heat olive oil in a large skillet over medium heat, then sear the chicken for about 8 minutes until golden. Remove from the skillet and set aside.
Add butter to the skillet, then toss in the diced red onions. Cook slowly on medium-low heat, stirring occasionally until they turn soft, golden, and sweet. This step takes around 15 minutes but builds the deepest flavor base.
Once the onions are ready, add sliced mushrooms and garlic. Stir gently and let the mushrooms brown as they release their juices. This gives the sauce its earthy, rich body.
Pour Marsala wine into the skillet, scraping up any browned bits from the bottom. Let it reduce slightly for about 5 minutes. Stir in heavy cream and sour cream, then lower the heat so the sauce stays silky without curdling.
Meanwhile, boil the pasta until al dente. Drain, then add it along with the cooked chicken into the skillet. Toss everything together so the sauce coats each piece of pasta. Garnish with fresh parsley before serving warm.