In medium bowl, whisk together balsamic vinegar, olive oil, mashed garlic, brown sugar, 1/4 cup fresh mint and 1 teaspoon salt. Pour ingredients into large zipper-lock bag; add chicken and seal bag. Turn chicken to make certain that all parts are coated with marinade. Refrigerate for at least 2 hours and up to 24 hours.

To cook chicken, place large skillet on stove over high heat. When very hot, add 1 tablespoon olive oil. Remove chicken from marinade and let excess drip off. Add chicken to skillet and saute, turning, until both sides are nicely browned and chicken is cooked through, about 5 - 7 minutes per side. Transfer chicken to plate and keep warm.

In large bowl, toss spinach, fennel bulb, grapes and 1/4 cup fresh mint. In small bowl, whisk together 1/4 cup olive oil, red wine vinegar, minced garlic, shallots, 1/4 teaspoon salt and black pepper. Pour vinaigrette over slaw and toss well to coat. Divide slaw among four plates. Cut chicken into long, thin strips and arrange on top of slaw. Garnish with additional mint slices, if desired.