1. Heat butter in large saucepan; cook garlic, stirring, 1 minute. Add chicken to pan; cook on both sides until browned lightly and cooked through. Remove chicken from pan.
2. Add stock to pan; bring to a boil, stirring. Reduce heat; simmer, uncovered, 5 minutes. Stir in mustard and tomato. Stir in onion.
3. Serve sliced chicken with sauce; accompany with roasted potato slices, if desired.