Caramelised onions: Heat 1 tablespoon olive oil in a large frying pan over low heat. Add onions. Cook, stirring occasionally, for 30 minutes or until soft. Add brown sugar and balsamic vinegar. Cook, stirring, for 5 minutes.
Heat oil in a frying pan over medium-high heat. Add chicken. Cook for 2 minutes each side or until golden. Remove to a plate.
Add fennel, thyme and garlic to pan. Cook, stirring, for 5 minutes or until soft. Add wine and mustard. Cook for 3 minutes. Add sweet potato and stock. Bring to the boil. Return chicken to pan. Cover and reduce heat to low. Simmer for 30 minutes.
Add cream. Cook, stirring, for 5 minutes (do not return to the boil as sauce might spit). Stir in caramelised onions. Serve.
To freeze: Allow to cool for 1 hour. Spoon into a 4 cup-capacity airtight container. Freeze for up to 3 months.
To reheat: Thaw in the fridge for 24 hours. Microwave, uncovered, on MEDIUM (50%) for 15 to 18 minutes, stirring every 5 minutes, or until heated through.