Heat your olive oil in a pan. Add onion and garlic and fry off before adding beef. Brown well. Add spices and salt. Fry a moment longer. Add tomato passata and turn the heat down-cook for 10 minutes on low. Remove from heat and cool completely.
Place the mince in a bowl and mix with mozzarella and cheddar.
Slice your onions into thick rings and separate into individual rings. Pop them on a lined baking tray and fill with the mince mix. Smooth over so the mince is level with the top of the piece of onion. Freeze until solid.
Bring out, then flour, egg and breadcrumb each stuffed ring.