1. Preheat the oven to 350°F.
2. In a skillet, heat the olive oil. Add the onions and cook over medium heat for 5 minutes. Add the brown sugar and simmer over low heat, covered, for 5 minutes.
3. Remove from heat and add the lemon zest. Mix.
4. Cut the cutlets in half lengthwise, taking care to keep them attached. Lay them flat on a cutting board. Season with salt and pepper on the inside.
5. Divide the onions among the 4 cutlets.
6. Close the cutlets with a toothpick.
7. Place the chicken on a baking sheet. Cover with aluminum foil.
8. Bake for 25 minutes.
9. Remove from the oven and serve hot.