Preheat the oven to 375°F (190°C). Spray a 9x13-inch baking pan with nonstick cooking spray.
Place 4 boneless skinless chicken breasts in a single layer in the prepared pan. Scatter 3 cups broccoli florets around the chicken.
Zest 1 medium lemon and set the zest aside for after the chicken has baked. Juice the lemon and place it in a small bowl. Add in 2 tablespoons olive oil, 3 tablespoons salted butter (melted), 4 cloves minced garlic, 1 teaspoon lemon pepper, and 1/2 teaspoon salt.
Pour the garlic-lemon mixture evenly over the chicken and broccoli.
Bake in the preheated oven for 35-40 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the broccoli is tender.
Remove the pan from the oven. Sprinkle the lemon zest and 1/2 cup grated Parmesan cheese over the chicken and broccoli. Serve hot, adding additional salt and black pepper to taste, if desired.