Prep the chicken. Pat the chicken dry and season with a light pinch of salt and pepper. Drying helps you get good browning.
Sear the chicken. Heat the olive oil in a large skillet over medium-high. Add the chicken in a single layer and cook 4–6 minutes, stirring once or twice, until mostly cooked and lightly golden.
Transfer to a plate.
Cook the zucchini. In the same pan, add a drizzle more oil if needed. Add zucchini with a pinch of salt and cook 3–4 minutes until just tender and slightly caramelized. Remove and set aside with the chicken.
Sauté the garlic. Lower the heat to medium.
Add garlic and stir for 30 seconds until fragrant. Don’t let it brown.
Build the sauce. Whisk in the chicken broth and ranch seasoning. Scrape up any browned bits.
Simmer 2 minutes to meld flavors.
Make it creamy. Stir in the heavy cream and Dijon. Bring to a gentle simmer and cook 2–3 minutes until slightly thickened.
Add cheese and lemon. Lower the heat and stir in Parmesan until melted and smooth. Add lemon juice.
Taste and adjust salt and pepper.
Combine. Return the chicken and zucchini to the skillet, along with any juices. Simmer 2–3 minutes, stirring, until the chicken is fully cooked and the sauce clings nicely.
Finish and serve. Sprinkle with chopped parsley or chives. Serve over rice, pasta, or mashed potatoes, or enjoy as is for a lighter option.