Cook fettuccine in pot of salted water, according to package instructions then drain and put aside
While the pasta is cooking, slice the chicken into strips and season all over with salt and pepper. In a large pan, heat olive oil over medium/high heat and saute chicken until lightly golden and cooked through. Remove chicken from pan and place aside and cover
In the same pan over medium/high heat add olive oil and butter. Then add onion and garlic and saute until soft. Add sliced mushrooms and saute until lightly browned.
Add heavy cream and simmer over medium/high heat for about 8 minutes, or until beginning to thicken. Season with salt and pepper
Add chicken back to the pan, and add the parsley
Add the cooked and drained pasta and stir to combine.
Cover and let rest for 10 minutes, then give it a quick toss and serve