Combine sausagemeat, onions, breadcrumbs, and egg in a bowl and season.
Roll the mixture into small balls using floured hands.
Heat oil in a pan and fry meatballs for 8-10 minutes until brown and cooked.
Boil potato pieces in salted water for 8 minutes.
Add peas and cook for 3 minutes until tender.
Drain vegetables and mash with mint, butter, and seasoning.
Sauté red onion in oil for 4 minutes, then add brown sugar for 2 minutes.
Add red wine and balsamic vinegar then simmer for 5 minutes.
Whisk in flour and caster sugar, season, and cook for 10 minutes until reduced.
Serve meatballs over the mash and top with gravy.