Split chicken breasts in half lengthwise and dredge in flour; remove excess.
In a large, heavy nonstick skillet, heat oil over medium-high heat. Cook chicken breasts on one side about 2 to 3 minutes, or until golden.
Add garlic. Turn chicken and continue cooking about 2 to 3 minutes, or until golden.
Add stock, balsamic vinegar and pepper. Reduce heat to medium-low, cover skillet and cook 5 to 10 minutes, or until chicken is tender.
Remove chicken from skillet. Keep warm.
In a small bowl, dissolve cornstarch in water, stirring until smooth.
Add cornstarch mixture to skillet and cook 1 to 2 minutes, or until thick and smooth.
Pour sauce over chicken and serve immediately.