Cook the pasta: Bring a large pot of salted water to a boil. Cook pasta until just al dente. Drain, reserving ½ cup of pasta water, and set aside.

Sear the chicken: Heat olive oil and 1 tablespoon butter in the same pot over medium-high heat. Add chicken, season with salt and pepper, and cook until golden brown and nearly cooked through, about 6–8 minutes. Transfer to a plate.

Sauté vegetables: Add remaining butter to the pot. Cook onions until soft, about 3 minutes. Add mushrooms and sauté until golden and tender, about 5 minutes. Stir in garlic and cook 1 minute more.

Deglaze with wine: Pour in the wine (or broth), scraping up browned bits. Simmer 2–3 minutes until slightly reduced.

Make the sauce: Stir in heavy cream. Slowly add the flour slurry, whisking to prevent lumps. Season with more salt, pepper, and garlic powder (if using). Simmer until thickened.

Combine everything: Return chicken and juices to the pot. Simmer until chicken is fully cooked. Add pasta and toss until coated in the creamy sauce. Loosen with reserved pasta water if needed.

Finish & serve: Remove from heat, sprinkle with parsley, and serve warm. Add Parmesan and red pepper flakes if desired.