Butterfly the chicken breasts by cut the breasts lengthwise 3/4 of the way through. Start at the tip and be sure to keep the knife level while slicing through the chicken breast.
In a small bowl, mix together the cheese filling: cream cheese, parmesan cheese, garlic, onion, salt, thyme, and lemon juice.
Stuff each butterflied chicken breast with half of the stuffing mixture leaving a 1/2" margin along the outer edge. Sprinkle the filling with fresh parsley.
Prepare a baking sheet or casserole dish by lining it with parchment paper or spraying it with cooking oil. Place the stuffed chicken breasts on the baking sheet.
Mix together the melted butter and lemon zest. Brush the tops of the chicken breasts with melted butter mixture.
Bake the cheese stuffed chicken breasts at 425ºF for 30 minutes. After 20 minutes, use a digital instant read thermometer to check the temperature of the chicken, then baste the chicken breasts to help them brown better. Chicken is done when the internal temperature in the center of the thickest portion is at least 165ºF.
Let chicken rest for 5 minutes. Before serving, cut each breast in half crosswise for serving. Serve with your favorite sides.We recommend mashed potatoes and asparagus or green beans, or a garden salad.
Notes
The average size of boneless chicken breasts varies a lot. On average, one breast can weigh anywhere from 6 to 12 ounces. If the chicken breasts you plan to cook weigh less than 6 ounces, you can double the amount of herb cheese filling and use 1 chicken breast per person.