Gather the ingredients.

Heat olive oil in a large skillet over medium heat. Add onion; sauté until translucent, 3 to 4 minutes.

Add chicken to skillet; cook until lightly browned, 1 to 2 minutes per side.

Combine ketchup, sugar, soy sauce, lemon juice, and pepper in a small bowl; pour over chicken. Bring to a boil, then reduce heat.

Cover and simmer until chicken is no longer pink in the center and the juices run clear, 25 to 35 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).