Place chicken breasts in a single layer between 2 sheets of heavy-duty plastic wrap; pound with a meat mallet or heavy skillet until chicken breasts are about 3/4 inch thick.
Heat oil in a 12-inch skillet over medium-high. Combine cumin, salt, and pepper; sprinkle evenly over both sides of chicken breasts. Add chicken to skillet; cook until done, about 4 minutes per side. Transfer to a cutting board; let stand 5 minutes. Cut across the grain into slices. Garnish with parsley, if desired.