Heat a 10-inch cast iron skillet over medium heat for about 5 minutes, then pour in the olive oil.
Season each halved chicken breast with 1 tablespoon of the ranch seasoning.
Transfer the seasoned chicken to the hot skillet, searing until browned on all sides and fully cooked through. Remove the chicken and set it aside on a plate, leaving the stovetop on.
Into the same skillet, add the butter along with chopped onions and minced garlic. Sauté over low to medium heat until the onions turn soft and translucent.
Introduce the broccoli florets to the pan, cooking until they are slightly charred to enhance flavor.
Stir in the chicken stock, ranch dressing, and heavy cream, mixing thoroughly.
Cover the skillet and allow the mixture to simmer until the broccoli becomes tender.
Return the cooked chicken to the pan, submerging it into the creamy mixture. Simmer until the sauce thickens and bubbles.
Finish by sprinkling grated Parmesan cheese over the top. Serve immediately.