To make the meatballs, mix together the onion, meat, thyme, breadcrumbs and milk in a bowl, and season well. Using a dessertspoon and one hand dipped in cold water (to stop it sticking), shape about 20 slightly oval balls out of the meat mixture. Place on a baking tray lined with baking paper, cover with plastic wrap and chill for 10 minutes, or overnight if time permits.
Preheat the oven to 350°F Heat the olive oil and butter in a large frying pan over a medium heat. Add the meatballs, in batches if necessary, and brown until golden all over. Transfer to an ovenproof dish and place in the oven for 10 minutes until cooked through.
While the meatballs are in the oven, take the frying pan in which you cooked them and place it over a medium heat. Whisk in the flour, add the tomato paste and whisk again for 1-2 minutes, then add the red wine. Turn the heat up to high and bubble for a few minutes. Whisk in the broth and simmer for 5-10 minutes. Season to taste with the Worcestershire sauce, salt and black pepper, then stir in a splash of cream and the redcurrant jelly.
(If freezing: Allow everything to cool, then tip the meatballs into a freezer bag and cover completely with the sauce. Freeze for up to 1 month.
Defrost completely and reheat in a saucepan until piping hot throughout.)Serve 3-4 meatballs per person with lots of sauce, spiced red cabbage and mash.