Heat oil in large oven proof frying pan

Cook onions, stirring, until soft.

Add the meatballs, cook for 5 mins stirring occasionally until browned all over.

Add tomatoes/passata. Cook for 5 min, stirring occasionally until meatballs are cooked through.

Remove from heat and stir in the spinach.

Arrange sliced mozzarella over the pan.

Preheat grill on high. Place frying pan under the grill for 3 min or until mozzarella melts and becomes golden brown.