Let chicken stand at room temperature for 20 minutes. Sprinkle chicken with 1/4 teaspoon pepper and 1/4 teaspoon salt.
Heat a large stainless-steel skillet over medium-high heat. Add 1 tablespoon oil; swirl to coat. Place chicken in pan, rounded side down, and cook for 5 minutes. Turn chicken over; reduce heat to medium, and cook 5 minutes more, or until done. Remove chicken from pan; let stand 5 minutes on a plate or platter covered with foil.
Add remaining oil, onion, and garlic to pan; sauté 1 minute. Next, add flour and sauté 30 seconds. Stir in stock; bring to a boil, scraping pan to loosen browned bits. Reduce heat to medium-low; simmer 3 minutes or until reduced to 1/2 cup, stirring occasionally. Remove pan from heat; stir in remaining pepper, salt, butter, and sugar. Sprinkle with parsley before serving.