Prep the chicken. Pat the chicken dry and season with a light pinch of salt and pepper.
Cutting the pieces evenly helps them cook at the same rate.
Brown the chicken. Heat olive oil in a large skillet over medium-high. Add the chicken in a single layer and cook, stirring once or twice, until lightly browned and just cooked through, about 5–7 minutes. Transfer to a plate and keep warm.
Sauté the aromatics. Lower heat to medium.
If the pan looks dry, add a drizzle more oil. Add garlic and cook about 30 seconds until fragrant. Don’t let it burn.
Build the sauce base. Sprinkle in the ranch seasoning, then pour in the chicken broth.
Stir, scraping up any browned bits. Let it simmer 1–2 minutes to bloom the spices.
Add the cream. Stir in heavy cream (and cream cheese if using). Simmer gently until the sauce starts to thicken slightly, about 3–4 minutes.
If you want it thicker, stir in the cornstarch slurry and simmer 1 more minute.
Cook the broccoli. Add the broccoli to the skillet and toss to coat in the sauce. Cover and simmer on medium-low for 4–5 minutes, until crisp-tender. If using frozen broccoli, it may cook a bit faster.
Finish the dish. Return the chicken and any juices to the skillet.
Stir in Parmesan. Simmer 1–2 minutes until the chicken is hot and the sauce is silky. Adjust salt, pepper, and red pepper flakes to taste.
Serve. Garnish with parsley or chives.
Spoon over rice, pasta, or mashed potatoes, or serve with warm crusty bread.