For the marinade: combine rosemary, lemon juice, lemon zest, 2 tablespoons of olive oil, garlic, salt and pepper in a large zip-lock plastic bag and mix well. Add chicken breast halves, seal bag and marinate at room temperature for 30 minutes, turning the bag once or twice.
Preheat oven to 425 degrees F.
In a large cast-iron skillet heat 1 tablespoon of the olive oil over medium-high heat. Add sweet potatoes and cook for 5 minutes, until the potatoes start to become tender. Stir frequently to avoid burning. Remove from heat.
Arrange chicken breast halves and lemon slices evenly in the same pan. Drizzle some of the remaining marinade over the sweet potatoes. Bake uncovered for about 40-45 minutes, or until chicken and potatoes are fully cooked. Enjoy!