Step 1: Cook the Pasta

1. Bring 4 quarts of salted water to a boil.
2. Add fettuccine and cook according to package directions until al dente.
3. Drain and set aside, reserving ½ cup of pasta water if needed for the sauce.

Step 2: Cook the Chicken

1. Slice chicken into even strips and season with salt and pepper.
2. Heat 2 tablespoons of olive oil in a large skillet over medium-high heat.
3. Add chicken and sauté for 5-6 minutes, turning occasionally, until golden brown and fully cooked.
4. Transfer chicken to a plate and cover to keep warm.

Step 3: Sauté Vegetables

1. In the same skillet, add 1 tablespoon olive oil and 1 tablespoon butter.
2. Add chopped onion and sauté for 3 minutes until soft.
3. Stir in sliced mushrooms and cook for 5-7 minutes until browned and tender.
4. Add minced garlic and cook for 30 seconds, stirring constantly.

Step 4: Prepare the Alfredo Sauce

1. Pour in half-and-half (or milk & cream mixture).
2. Bring to a gentle simmer over medium heat for 8-10 minutes, stirring occasionally.
3. Stir in Parmesan cheese, letting it melt into the sauce.
4. If the sauce is too thick, add a bit of reserved pasta water to loosen it.
5. Season with salt and black pepper to taste.

Step 5: Combine Everything

1. Return cooked chicken to the pan.
2. Stir in ¼ cup chopped parsley for freshness.
3. Add drained fettuccine pasta to the sauce and toss until fully coated.
4. Heat for 1 more minute, stirring continuously.

Step 6: Rest & Serve

1. Turn off the heat, cover the skillet, and let sit for 10-15 minutes to allow flavors to meld.
2. Stir once more before serving.
3. Garnish with extra parsley and Parmesan cheese.