Heat olive oil in a large skillet over medium high heat.
Season chicken with 1/2 teaspoon salt and 1/2 teaspoon pepper.
Add chicken to skillet and brown on each side--about 2 minutes per side.
Add garlic, green beans, and tomatoes to skillet and saute for 1-2 minutes.
Add in chicken broth, zest of lemon, thyme sprig and balsamic vinegar and cover skillet. Let cook about 5 minutes or until chicken is cooked through.
Right before serving, remove thyme sprig, squeeze with fresh lemon juice, and sprinkle with Parmesan if using.
To keep the green beans crisp-tender, I recommend blanching them first. This can be done by bringing a pot of salted water to a boil and dropping green beans into the boiling water for 2 minutes. Remove green beans and place in an ice bath to stop the cooking process and drain before adding to skillet.
If you don't want to take the time to blanch the green beans first, just place them in a microwave safe bowl with 1 tablespoon of water. Cover with plastic wrap and microwave for 3-4 minutes.
You can skip the step of steaming or blanching your green beans first, they will just be crunchier in the skillet.
The fresh thyme is not necessary, but does had a nice flavor.