1-First Step: Gather and prep everything
Start by getting all your ingredients ready before you turn on the stove. Chop the onion, slice the cremini mushrooms, cut the broccoli florets into bite-size pieces if needed, and remove the rotisserie chicken breast from the bone. Slice or chop the chicken into small pieces so it blends easily into the pasta later.
At this point, set a pot of water on the stove for the pasta. Bring it to a boil and cook the pasta according to the package directions. While that cooks, you can work on the skillet portion of the dish. This is a great time-saver because the sauce and pasta finish close to the same time.

2-Second Step: Cook the onion in butter
Place a large skillet over medium heat and add the 3 tablespoons of butter. Once the butter melts, add the chopped onion. Cook the onion for about 3 minutes, stirring now and then, until it turns soft and fragrant. You do not want it browned here, just tender and lightly sweet.
This first step builds the base flavor of the dish. Butter and onion give the sauce a rich, savory start that works well with the chicken and cream later on. If the pan seems too hot, lower the heat a little so the onion cooks gently instead of getting too dark.

3-Third Step: Add the mushrooms and broccoli
Now add the sliced mushrooms and broccoli florets to the skillet. Cook them for about 2 minutes, stirring so they get coated in the butter and onion mixture. The goal is to soften them slightly while keeping the broccoli bright and the mushrooms tender.
If you like your broccoli very soft, you can cook it a little longer, but be careful not to let it lose its color. A short cooking time helps keep the vegetables fresh and pleasant to eat in every bite.

4-Fourth Step: Stir in the chicken and cream
Add the sliced rotisserie chicken breast to the pan, then pour in the 1 cup of cream. Stir everything together so the chicken and vegetables are fully coated. Bring the mixture to a boil, then lower the heat to medium and cook for another 3 minutes.
This brief simmer gives the chicken time to warm through without drying out. Since rotisserie chicken is already cooked, you mainly want to heat it gently and let it soak up some of the sauce flavor. If you are using leftover chicken thighs or drumsticks, remove the meat from the bone first and chop it into bite-size pieces before adding it to the skillet.

5-Fifth Step: Add the cheese and pasta
When the sauce has simmered and slightly thickened, reduce the heat and stir in the 1/2 cup freshly grated Parmesan cheese. Add the cooked pasta to the skillet and mix until everything is well combined. The cheese should melt into the cream and create a glossy sauce that clings to the pasta.
Add the 1/8 teaspoon salt and freshly grated pepper to taste. Taste the pasta before adding too much extra salt, since Parmesan and rotisserie chicken can already bring some saltiness. If the sauce seems too thick, add a small splash of pasta water to loosen it up.

6-Final Step: Serve and finish with garnish
Spoon the pasta into bowls or onto plates while it is still warm. Serve it garnished with chopped green onions and extra Parmesan cheese, just like the recipe calls for. The green onions add a fresh, mild bite that balances the rich sauce and gives the dish a nice finished look.
For the best texture, serve it right away. The sauce is creamiest when it is freshly mixed, and the vegetables stay at their best while warm. If you want a little extra richness, a final sprinkle of Parmesan on top is always a good idea.